A president doesn't take to Twitter to engage in a stream of thought speculation after a terrorist attack.
"I spoke with a wonderful woman, British Prime Minster Theresa May, this morning and relayed America's deepest sympathy as well as our absolute commitment to eradicate the terrorists from our planet", Trump said.
"We have to be tougher and we have to be smarter", Trump told reporters before going into the Oval Office.
The president also used the attack to promote his travel ban that is aimed at temporarily barring people from six Muslim-majority countries from entering the U.S.
About 22 people were injured in the bomb black that rocked a London Underground train on Friday.
The British Prime Minister, who described the attack involving an improvised explosive device (IED) on a Tube train at Parsons Green station as a "cowardly" act "clearly meant to cause significant harm", was addressing the media after chairing an emergency response COBRA committee meeting at Downing Street.
An 18-year-old man was arrested on Saturday in connection with the attack, though police have not identified or charged him.More news: Luke Shaw, Paul Pogba and Marcos Rojo Injury Updates
"The travel ban into the United States should be far larger, tougher and more specific - but stupidly, that would not be politically correct", the president said in a series of tweets. He adds: "Must be proactive & nasty!"
The President has form with London attacks.
On Friday, he again stressed that the internet was a main recruitment tool for terrorists. Along with advocating for a "larger, tougher" travel ban, the President also suggested shutting down the internet.
In an early-morning string of tweets, Trump appeared to criticize British authorities for their handling of the strike and insinuating they knew of the alleged assailant. "Must be proactive!", Trump said through his Twitter account.
Recently, the Supreme Court allowed the Trump administration to maintain its ban on refugees.
As for the travel ban, A.P. notes that remains tangled in the court systems with the Supreme Court scheduled to listen to arguments on October 10. He and his son Donald Trump Jr both mocked Sadiq Khan after taking the mayor's words of reassurance to Londoners out of context.