Why is there still no US envoy to UK?
The war of words between US President Donald Trump and London Mayor Sadiq Khan following Saturday's terror attack in the UK has exposed chinks in the so-called "special relationship".
The US Embassy in the UK has not been singing from the same hymn sheet as the White House in recent days as it tries to soften the blow of Mr Trump's rebuke of how Khan has handled the London Bridge attacks.
While Mr Trump branded Khan's appeal for calm a "pathetic excuse" on social media, America's top diplomat to the Court of St James's - Chargé d'affaires ad interim and former Deputy Chief of Mission Lewis Lukens - praised the mayor's leadership.
The lack of co-ordination between the Trump administration and the US Foreign Service transpired without an ambassador spearheading America's diplomatic efforts in the country.
So how is the lack of a top US diplomat playing out in London?
The ambassador traditionally advocates for American interests in the UK, deciphers British politics and policies for the federal government and offers consular services to citizens, according to former US Ambassador to the Czech Republic, John Shattuck.
In times of crisis, however, the ambassador becomes a fulcrum between the two countries as they take part in intelligence sharing meetings and strategic planning sessions around cross-national issues like security.
"An ambassador, fully accredited by their country, may be able to enter meetings at a higher level than lower, professional staff," says Mr Shattuck, now a professor at Tufts University.
Without a proper ambassador in place, the flow of information could be impaired, he says, "but that's not a foregone conclusion. A second-in-command may well be given access."
After events like the London Bridge attack, an ambassador would usually express US sympathy and solidarity and ensure affected Americans were cared for - as Chargé d'affaires Lukens did, says W Robert Pearson, former US Ambassador to Turkey.
What was potentially missing at the weekend was an ambassador who was able to immediately reach high-ranking officials at the White House, the National Security Council or the Department of State to manage communications and set a tone moving forward.
"With such work in the moments after the tragedy, the transatlantic conversation might not have ended up characterised as an exchange between the president and Mayor Kahn, which now has gone so far as to bring about a rebuke of Mr Trump's comments from Prime Minister May," Pearson says.
"Not to put too fine a point on it, that missing piece allowed the exchanges to end up obscuring the message of solidarity which the American people would have wanted to convey, was owed to the UK and which the UK would have welcomed."
Still, there are limits to an ambassador's influence, especially in the age of Trump.
"How much that person would have been able to do to manage the intelligence leaks we'll never know," Mr Pearson says.
"To the White House, I doubt that having an ambassador present would have persuaded them to make maximum use of the embassy at such a critical moment."
The post has remained vacant since former Ambassador Matthew Barzun left on 18 January.
Mr Trump lashed out at Senate Democrats on Monday via Twitter for stalling confirmation hearings that would appoint key people to his administration, including ambassadors.
However, despite stating his intention to nominate businessman and New York Jets football team owner Woody Johnson as UK Ambassador, Trump has not yet done so - meaning there is no one for the Senate to confirm.
An official from the US Department of State referred queries about nominations to the White House, but said embassy and consular staff abroad were ready to help Americans in need of assistance.
Although gaps in foreign service coverage are not unusual during presidential transitions, the problem has been exacerbated by holes in the State Department's upper echelons, according to Johns Hopkins School of Advanced International Studies (SAIS) Professor Erik Jones - including the absence of a permanent Assistant Secretary of State for European and Eurasian Affairs.
While missions can operate under a chargé d'affaires, Jones said the White House was "not in sync" with its diplomatic corps thanks to the limited number of political appointees, as opposed to career professionals, who align with the president's views and can form effective conduits for his priorities.
"That doesn't mean we don't have organisation," Mr Jones says. "It means we don't have a high-profile representative who is personally close to the president."
But it remains to be seen whether "hurt feelings and miscommunication" translate into institutional decisions with serious ramifications, such as a vacuum of power on the world stage, Mr Jones says.
"I think the obvious question is whether a president acting alone is capable of having a foreign policy - a complex beast that builds on multiple and overlapping relationships - and it seems as though one person cannot have all these relationships at the same time effectively," Jones says.
It is unclear if an ambassador will be appointed before Trump makes a landmark state visit to the UK, slated for later this year.