Mark Rowley, assistant police commissioner for London's Metropolitan Police, commonly referred to as Scotland Yard, said the public would see a heightened police presence at transport hubs and urged anyone with images of the incident, or knowledge of the attack, to contact authorities.
Minutes later, the USA president followed with two more tweets, commenting on the fight against terrorism and his administration's efforts to do so.
"I've simply been too busy this morning to look at Twitter", Khan said when asked about Trump's tweets in an interview aired on a British radio station.
11-year-old boy meets President Trump, mows White House lawn
Trump introduced Frank to the assembled media and then invited him (and his father) into the Oval Office . "We're lucky. When Frank Giaccio wrote to President Trump, he said it would be an honor to mow the lawn at the White House.
A bucket wrapped in an insulated bag caught fire on a packed London subway train Friday, sending commuters stampeding in panic at the height of the morning rush hour.
The president has repeatedly criticized London Mayor Sadiq Khan, the British capital's first Muslim leader.
Prime Minister Theresa May said after the attack that the country's Joint Terrorism Analysis Center made a decision to raise the country's threat level to critical, meaning that a further attack may be imminent.
So far this year the United Kingdom has had a series of unprecedented terror attacks in Westminster, London Bridge, Borough Market, Manchester and Finsbury Park.
UN Chief calls for effective action plan to address Rohingya problem
The Rohingya are fleeing from a Myanmar military offensive in Rakhine that began after a series of guerrilla attacks on August 25 on security posts and an army camp.
"This is a proportionate and sensible step which will provide extra reassurance and protection while the investigation progresses", she said.
Police said the incident could have been much worse, had the man-made explosion device worked to its full potential. "As London has proven again and again, we will never be intimidated or defeated by terrorism", he said. "We have to be tougher and we have to be smarter".
The travel ban bars certain people from Iran, Libya, Somalia, Sudan, Syria and Yemen from entering the US.
Under the order, foreign nationals are banned for 90 days from six countries - Iran, Syria, Sudan, Somalia, Libya, and Yemen - and refugees lacking a link to the USA are barred for 120 days.
Why? Robert Hodges kills West Sacramento wife's three children
Alina and Daniel Vertelnikov, who are friends of the victims, recalled how happy the kids seemed. " It just happened. The mother of the children is considered a domestic violence victim, according to the police.
The issue concerning the scope of the travel ban has been making its way through the courts since last spring, when the Supreme Court allowed Trump's ban to go into effect except for those with a "bona fide" relationship to the United States.