In November, the BBC took a look into what steps the US takes when admitting Syrian refugees to the country.
- Forty-nine people killed in attack on gay nightclub - the worst mass shooting in recent US history
- Suspect took hostages and died in gunfight with Swat officers
- He is named as 29-year-old Omar Mateen, US national
- Mateen investigated twice by FBI since 2013, but was not on active terrorism watch list
- US President Obama says no evidence he was part of a larger plot
The New York businessman has now moved to attacking Hillary Clinton by criticising her statements after the San Bernadino attacks in California.
"I will always support the second amendment," he says.
Trump also claims Obama "is not allowing" law enforcement to do their job and suggests the US needs an "intelligence operation second to none".
In a speech in New Hampshire, Republican Donald Trump says the US must respond to "force, purpose and determination" and adds: "If we don't get tough, we're not going to have a country any more."
Mr Trump has also reiterated his call for a ban on all Muslim immigration, referencing Mateen's father's immigration as the "only reason this killer was in the country".
Vigils are being held across the UK in memory of the 49 people killed at a gay nightclub, including this one in Soho in London.
Makeshift memorials have also cropped up near US embassies in other countries.
In her first speech since the Orlando shootings, Democratic presidential candidate Hillary Clinton says the country faces a "twisted ideology and a poisoned psychology" that inspires "lone-wolf" attacks.
Vowing to make stopping these kind of solo attacks a priority of her presidency, she added: "If you are too dangerous to get on a plane, you are too dangerous to buy a gun in America."
Former American Idol singer Clay Aiken, also a Democratic politician, says the Orlando shooting victims should be honoured by improved gay rights in the US.
Gay men and women were persecuted before Sunday, he told the BBC. For example, he said, they can still lose their jobs for being gay.
FBI director James Comey said he was "highly confident this killer was radicalised" and partially through the internet.
More from Comey:
- Mateen questioned in 2013 because he made "inflammatory and contradictory" statements, including claiming connections to both al-Qaeda and Hezbollah - two groups diametrically opposed to each other
- he told the FBI he had made the comments in anger because he thought colleagues were discriminatory and mocking him
- the FBI followed him and investigated further but closed the case after 10 months
- second investigation began because Mateen once attended the same mosque as a suicide bomber, and a person the FBI interviewed had once been concerned that he had been radicalised, but was no longer worried because he had been recently married and had a child.
Mr Obama said the investigation was in the "preliminary stage" but is being treated as a terrorist investigation.
It appears Omar Mateen was able to buy the weapons he used in the attacks legally - as he had no criminal record, Mr Obama said, and was able to carry one gun "out of the store".
He said "this is something we are going to grapple with" while targeting so-called Islamic State abroad.
President Obama said: "If we have self-radicalised individuals in this country.. they are going to be very difficult to find ahead of time."
"How easy they can get weapons is going to make a difference about how they can carry out these attacks... its a problem regardless of motivations." Mr Obama said, referencing last years' shootings at a church in Charlestons and more recent shootings at UCLA.
In brief remarks in the Oval Office after being briefed by the FBI, President Barack Obama has said there is no evidence the Orlando nightclub killer was part of a larger plot.
He says the killer, Omar Mateen, appeared to have been inspired by extremist information online, describing the attack as home-grown extremism.
He said weak gun laws in the US meant it was too easy for people to get hold of powerful rifles.
A crowdfunding effort for the families of victims run by Equality Florida has surpassed $1.5m, reacing the $1m mark faster than any in Go Fund Me history, according to former Obama adviser and Go Fund Me executive Dan Pfeiffer.
During a press conference with Cypriot Foreign Minister Ioannis Kasoulides, US Secretary of State John Kerry said "the worst thing we can do is engage in trying to point fingers at one group or one form of sectarianism or another or one religion or another".
"Those are not the values of our country," Kerry told reporters.
In a series of tweets, Orlando Health says no patients have died from the injuries since the initial nine on Sunday. Six patients have been discharged from hospital and 26 remain. Five remain in a "grave" condition and a number are still in shock.
Saudi Interior Ministry security spokesman Major General Mansour Turki has said Mateen performed the umrah Islamic pilgrimage for 10 days in March 2011, and eight days the following March.
Tributes to the victims have been paid across the world. A vigil was held at Frank Kits Park, in Wellington, New Zealand, in remembrance of victims.
Meanwhile, the Sydney Habour Bridge in Australia was illuminated in rainbow colours in tribute.
A vigil was also held in Seoul on Monday to honour the victims
The UK's home secretary, Theresa May, has described the attacks in Orlando as "utterly evil" and an "outrage committed to spread fear, born out of hatred".
She said the attack had its roots in a "twisted ideology, which counts homophobia as a cornerstone of its warped world view".
She said the UK was working closely with US authorities and no British nationals were involved in the attack. The UK will review plans for large scale events, she added.
Donald Trump's comments in the aftermath of the Orlando shootings have been branded simplistic and wrong by a former director of the CIA. Michael Hayden was reacting to the Republican presidential candidate saying thousands of people in America were "sick with hate" and Muslims knew who they were and should turn them in. (Photo: Donald Trump. Credit: Getty Images)
The Guardian's Spencer Ackerman reports the FBI investigated Mateen in 2013 after he claimed to colleagues at G4S to be associated with the Tamerlan and Dzhokhar Tsarnaev, the two brothers responsible for the Boston marathon bombings.
"Ultimately, bureau investigators determined that Mateen had invented the connection and did not pose a security threat," Ackerman writes.
The FBI confirmed the false claim but did not give any additional details, the newspaper reported.
Ivory Mcneal was at Pulse nightclub in Orlando, Florida, when a gunman killed six of his friends in the deadliest mass shooting in recent US history.
He texted his husband Jack - who was in the UK - as he hid from the killer.
Speaking at the US Institute of Peace in Washington, the Buddhist leader called on the audience to observe a moment of silence, calling the shootings a "very serious tragedy".
Police said there were two guns used (and a third left unused in the gunman's car). One was a 9mm handgun, the other a AR-15 semi-automatic rifle - the same weapon used in the mass shootings in Aurora, Colorado, in San Bernardino, California and in Sandy Hook, Connecticut.
Police did not specify what brand of AR-15, and how many round it held - the amount of rounds you can legally have in a gun differs from state to state.
Former Afghan President Hamid Karzai has supported new ways to address the issue of radicalisation.
Speaking to the BBC he said: "The question of radicalisation and extremism is a living condition in our societies. That needs to be addressed in ways different to what we are doing today."
Omar Mateen was a US citizen of Afghan descent, who was born in New York and lived in Florida and was not on a terrorism watch list.
But he was interviewed twice by the FBI in 2013-14 after he made "inflammatory remarks" to a colleague, before authorities closed the investigation.
Asked about radicalisation in Afghanistan, Mr Karzai said: "The arrival of radicalisation was the result of the war against against the Soviet Union.
"It has not come to us out of our social environment, but out of an environment of war [and foreign] interference".
In a statement, the Kingdom of Saudi Arabia says it "condemns in the strongest terms the attack on innocent people in Orlando, Florida" and "sends its deepest condolences to the families and friends of the victims".
Homosexuality is illegal in Saudi Arabia.
Meanwhile, NBC News is reporting Mateen visited Saudi Arabia in 2011 and 2012, but the purpose of the trips may have been to attend hajj.
Afghan social media users express mixed reactions to the Orlando attack by Afghan-American Omar Mateen with some approving of it and referring to him as a martyr, while others criticize his actions.
Several have posted on the official Facebook Page of the Afghan Pajhwok News agency beneath a message about the attack. User Khaled Zaker Omar says: "Congratulations on your martyrdom. May Allah give such feeling to other Afghans."
Mdawood Dawood under the same message, writes: "May God accept the young man's martyrdom."
But in a comment posted on the official Facebook page of Voice of America presenter Lina Rozbih-Haidari, user Anam Naibkhil writes: "He perhaps had mental disorder, like the American soldier who killed 16 women and children in Kandahar."
And user Syed Sadat says: "Death to this savage man. We had discussions yesterday to find out to which country the pride of Mawlana Rumi [famous poet and writer] belongs to, Afghanistan, Iran or Turkey. Now, who will take the responsibility of pride of this man, USA, Afghanistan, or Muslims?"
User Zamer Meraan Sanjanay says: "An ignoramus killed some people and some other ridiculous people say congratulations for his martyrdom."
The former head of the CIA, General Michael Hayden, has insisted that the intelligence agencies would have been unable to prevent the shootings in Orlando. The gunman Omar Mateen was interviewed three times by the FBI, but agents concluded he was not a threat. General Hayden - who also oversaw the National Security Agency - told World At One presenter Martha Kearney there was little that surveillance could have done to stop such attacks without changing the nature of a free society. (Photo: Rainbow flag is waved during a White House vigil for the victims. Credit: Reuters)
Daniel Gilroy, who worked with Omar Mateen at G4S, told multiple news outlets the man repeatedly made threats of violence.
"He talked about killing people all the time,'' he said.
"He was an angry person, violent in nature, and a bigot to almost every class of person."
Mr Gilroy said he reported Mateen's behavior to his bosses and eventually quit working with him.
"I kind of feel a little guilty that I didn't fight harder," he said. "If I didn't walk away and I fought, then maybe 50 people would still be alive today."
Here in the UK, the Archbishops of York and Canterbury have issued a joint statement calling on Christians to support lesbian, gay, bisexual, trans and intersex people.
"After Sunday's attack in Orlando, as Christians we must speak out in support of LGBTI people, who have become the latest group to be so brutally targeted by the forces of evil. We must pray, weep with those affected, support the bereaved, and love without qualification. The obligation to object to these acts of persecution, and to support those LGBTI people who are wickedly and cruelly killed and wounded, bereaved and traumatised, whether in Orlando or elsewhere, is an absolute call on our Christian discipleship."