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LONDON — Britain’s prime minister rebuked President Donald Trump on Friday after he tweeted that "a loser terrorist" responsible for a subway train detonation had been "in the sights" of London’s Metropolitan Police.

Asked whether Trump knew something the British public did not, Prime Minister Theresa May said, "I never think it's helpful for anybody to speculate on what is an ongoing investigation."

May's former chief of staff, Nick Timothy, went further, describing Trump's comment as "so unhelpful from leader of our ally and intelligence partner.”

True or not - and I'm sure he doesn't know - this is so unhelpful from leader of our ally and intelligence partner. https://t.co/ZndrTXFrAX — Nick Timothy (@NickJTimothy) September 15, 2017

Shortly after 29 people were injured in the rush-hour incident at Parsons Green in London, Trump tweeted: "Another attack in London by a loser terrorist. These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!"

His comments appeared to point the finger at British police, who had not released any information about suspects. It was not immediately clear how Trump obtained his information, or if he had revealed classified details.

Britain’s Police Federation, which represents officers, referred reporters to the Metropolitan Police.

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May announced on Friday night that the nation's threat level for international terrorism had been increased to critical, its highest level, meaning another terror attack may be imminent.

Police said no arrests had been made and would not comment on the manhunt for the culprit. London subway stations are equipped with surveillance cameras.

Multiple law enforcement officials briefed on the investigation said authorities in the U.K. had identified a person they believe is responsible for the attack. An active manhunt was underway Friday evening, they added.

ISIS made its customary claim of responsibility for the attack, without providing any supporting evidence.

Mayor Sadiq Khan assured the public that "everything possible is being done to protect our city and keep us all safe," adding that additional police officers would patrol the streets throughout the weekend.

Luke Walmsley, 33, who was six train cars up from the fire described “complete pandemonium, complete terror" aboard the train.

“We heard the first scream and we saw a flash and smoke and people started running immediately,” he told NBC News.

Walmsley said that several people suffered "severe brown burns" to their heads. "It was a really acrid burning smell," he added.

Charlie Craven, 29, said he boarded the train seconds before it ignited.

"A massive bang occurred, we didn’t know what happened," he said. "We looked around and the first thing we saw was an orange fireball ... coming toward you."

Craven said he managed to jump over a fence and run away down the train tracks. “There was this mass hysteria of people shouting, screaming trying to get away from the situation,” he added.

Related: Nannies, Parents Race to Find Their Kids After Subway Blast

In May, British authorities were angered when details about a deadly bomb attack at an Ariana Grande concert in Manchester were leaked to American media.

The attacker's identity and details of the investigation were revealed in U.S. media before British authorities made the information public, while The New York Times published photographs from the scene, as well as images of the bomb.

The U.K. has been the target of three previous deadly terror attacks this year. A total of 33 people have been killed in vehicle-and-knife rampages on London Bridge and nearby Borough Market as well as Westminster Bridge.

Trump was widely criticized for tweets in the wake of the June attack. He described Khan as “pathetic” for saying Londoners had “no reason to be alarmed” by the visible increase in armed police officers in response to the attack — apparently misinterpreting the mayor’s words as a comment on Islamic extremism.

London’s transit system was the target of a series of coordinated suicide bombings on July 7, 2005. Oftenreferred to as “7/7,” the attacks during the morning rush hour killed 52 people and wounded hundreds.