President Trump made clear Tuesday evening that the Islamic State is not welcome in the U.S., after saying a “sick and deranged” person committed the deadly truck attack in Manhattan earlier in the day.

“We must not allow ISIS to return, or enter, our country after defeating them in the Middle East and elsewhere. Enough!” Trump tweeted.

Police said a truck drove into a bike lane Tuesday afternoon and eventually hit a school bus, injuring two adults and two children. As the suspect drove the truck, reports say he shouted “Allahu Akbar,” translating to “God is Great!” in Arabic. The suspect has been identified as Sayfullo Habibullaevic Saipov, a 29-year-old man from Tampa, Fla.

There is no confirmation yet this was an ISIS-linked attack; however, just before Trump tweeted, House Homeland Security Committee Chairman Michael McCaul, R-Texas, said on Fox News he firmly believes it was orchestrated by the Islamic State. He was reacting to reports that the suspect yelled "Allahu Akbar." "This was the ninth vehicle assault used by ISIS," McCaul told Brett Baier on Fox News, referring to recent ISIS-linked attacks around the world in which vehicles were involved.

The U.S. and other countries are currently waging war against ISIS in the Middle East, particularly in Iraq and Syria, and while ISIS gets squeezed in the region, experts have said that ISIS-linked attacks elsewhere in the world can be expected.

"In NYC, looks like another attack by a very sick and deranged person. Law enforcement is following this closely. NOT IN THE U.S.A.!" Trump tweeted earlier on Tuesday.

In another tweet in which he referred to the "terrorist attack," Trump sent out his "thoughts, condolences and prayer" to the victims and families impacted by the incident.

At a media briefing Thursday afternoon, New York City Mayor Bill de Blasio also referred to the incident "an act of terror."

The suspect was shot by New York police after he got out of the truck with two handguns in tow, according to New York City Police Commissioner James O’Neill. A paintball gun and pellet gun were found at the scene.

Police said eight people were killed in the attack, and 15 people were injured. The suspect is now in custody.

An investigation is now underway, and Trump said in a statement that his administration "will provide its full support to the New York City Police Department, including through a joint investigation with the Federal Bureau of Investigation."

"We offer our thanks to the first responders who stopped the suspect and rendered immediate aid to the victims of this cowardly attack. These brave men and women embody the true American spirit of resilience and courage," Trump added. "I will continue to follow developments closely."