San Bernardino shooting: Policeman speaks of carnage at scene
One of the first police officers to respond to deadly shootings at a social service centre in California has spoken of scenes of "unspeakable" carnage.
Lt Mike Madden said he and officers who arrived later saw dead bodies and had to pass injured people as they tried to "engage the shooters" on Wednesday.
Officials say a husband and wife shot dead 14 people and wounded 21 in the city of San Bernardino.
Thousands attended a candlelit vigil in the city on Thursday evening.
Friends and relatives of the victims were among those present at the San Manuel stadium in downtown San Bernardino.
The lights were turned out as attendees waved candles and lights.
Suspects Syed Rizwan Farook, 28, and Tashfeen Malik, 27, were killed in a shootout with police.
Bomb equipment, weapons and thousands of rounds of ammunition were later found in the attackers' home.
San Bernardino shooting - in depth
Who carried out shootings? - The couple police say were responsible
What makes this shooting different? - More than one shooter, a woman involved, a well-planned attack, explosives and a fleeing attempt
'It's crazy they lived next door' - Neighbours tell the BBC of their shock that the attackers lived nearby
Politicians 'shamed' for offering prayers - Does prayer do anything in the wake of a shooting?
Who were the victims? - Diverse backgrounds of the 14 people killed
The authorities have still not found a motive for the attack, but the FBI has now taken over the investigation.
"It was unspeakable, the carnage that we were seeing," Lt Madden told a news conference.
"The number of people who were injured and unfortunately already dead and the pure panic on the face of those individuals that were still in need and needing to be safe."
He said he and other police officers led about 50 people out of the centre's conference room.
"Then we went further into the building and that was a difficult choice to have to make as well, passing people that we knew were injured and in need of assistance," Lt Madden said.
"But our goal at that time had to be trying to locate the shooters and deal with them."
Police said between 75-80 people were at the centre when the shooting began.
The names of the victims have now been released by San Bernardino's coroner.
The youngest victim was 26 and the oldest was 60.
Police said the attack indicated there had been "some degree of planning".
Local police chief Jarrod Burguan said it appeared that the duo was prepared to carry out another attack.
"There was obviously a mission here. We know that. We do not know why. We don't know if this was the intended target or if there was something that triggered him to do this immediately," said David Bowdich, assistant director of the FBI's Los Angeles office.
In the shootout with police hours after the attack, Farook and Malik fired 76 rounds of ammunition at the officers and the officers fired 380 rounds back.
Two police officers were injured during the pursuit.
Farook had been in contact with known Islamic radicals on social media, unnamed US intelligence officials said.
President Barack Obama said the FBI had taken over the investigation from local authorities.
"It is possible that this was terrorist-related, but we don't know. It's also possible that this was workplace-related," Mr Obama said.
The FBI cautioned that the authorities needed time to investigate.
San Bernardino is the deadliest mass shooting in the US since 26 people were killed at a school in Newtown, Connecticut in 2012.