Donald Trump's tweet claiming that the perpetrator behind an explosion on a Tube train was known to British security services is "speculation", police say.
"We don't even know who the suspects are so it's a bit difficult to say," a Metropolitan Police source told The Independent. "It's just speculation."
Theresa May later rebuked her ally following a meeting of the Government's Cobra emergency committee, saying: “I never think it's helpful for anybody to speculate on what is an ongoing investigation.''
The US President used the terror attack in Parsons Green to call for a "far larger, tougher" travel ban in the US.
“Another attack in London by loser terrorist,” Mr Trump tweeted. “These are sick and demented people who were in the sights of Scotland Yard.”
He appeared to be suggesting that the perpetrator or perpetrators were previously known to British security services, as was the case with the attackers in Westminster, Manchester and London Bridge.
Mr Trump went on to call for the internet to be “cut off” as a recruitment call for terrorists.
Police have not yet named a suspect or confirmed how many people are being sought in the wake of the attack, but said MI5 agents were involved in the ongoing investigation.
More than 20 people were injured when the improvised explosive device ignited on a District Line train at Parsons Green in the height of morning rush hour.
Witnesses described a ball of flame erupting in the end carriage, prompting a stampede to flee the station.
Images captured by commuters showed a small fire contained in a plastic bucket inside a shopping bag, with wires seen trailing from inside.
Sources told the BBC and Sky News the bomb, which ignited but failed to fully detonate, may have been attached to a timer.
Investigators were combing through CCTV footage from the London Underground and images taken by witnesses in efforts to discover who planted the device.
“This was a detonation of an improvised explosive device,” said Assistant Commissioner Mark Rowley, from the head of national counter-terrorism policing.
“There are many urgent inquiries ongoing with hundreds of detectives involved, looking at CCTV, carrying out forensic work and speaking to witnesses.”
He urged Londoners to be alert but not “alarmed” amid a heightened police presence in the capital, with the nationwide terror threat level remaining at “severe”.
The Prime Minister said the bomb was "intended to cause significant harm" and said her thoughts were with the victims, while thanking the emergency services for their bravery and professionalism.
"Police and the security services are obviously doing all work to discover the full circumstances of this cowardly attack and to identify all those responsible."
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