WASHINGTON D.C. -- President Trump has expressed his condolences to Great Britain after Saturday night's terrorist attack, but also took the opportunity to criticize London's mayor and promote his travel ban on Twitter.
However, security experts say no travel ban would have stopped this kind of attack.
"We must stop being politically correct and get down to the business of security for our people," the president tweeted on Sunday morning.
He then criticized London's mayor for saying there was "no reason to be alarmed." The mayor says he was trying to reassure Londoners who were seeing an increased police presence in the city.
In another tweet, President Trump said, "we need the travel ban as an extra level of safety."
But critics say that wouldn't have stopped this kind of attack and is not the answer.
"A travel ban is cannon fodder for the recruiters, it's the worst thing we can do," said former Secretary of State John Kerry.
Others fear stigmatizing Muslims from certain countries will backfire.
"We alienate the very communities here in the United States whose cooperation we most need to detect and prevent these homegrown extremists from being able to carry out attacks," said Susan Rice, former National Security Adviser.
Security experts say while the U.S. is at risk for this kind of attack, parts of Europe have much greater extremist populations.
"We've never had the same degree, the same volume, and the same speed of radicalization that the U.K., Belgium, and the French have had," said NBC News National Security Analyst Michael Leiter.
The Trump administration has asked the Supreme Court to reinstate the proposed travel ban on six mostly-Muslim countries. Lower courts have blocked the policy.