The gunman who took four people hostage at a Texas synagogue in a 10-hour standoff that ended in his death was checked against law enforcement databases before entering the U.S. but raised no red flags, the White House said Tuesday.

Malik Faisal Akram, a 44-year-old British citizen, arrived in the U.S. at Kennedy Airport in New York about two weeks ago, a law enforcement official said. He spent time in Dallas-area homeless shelters before the attack Saturday in the suburb of Colleyville.

Also Read: Texas synagogue hostage-taker Malik Faisal Akram was on MI5 watchlist

British media, including the Guardian, reported Tuesday that Akram was investigated by the domestic intelligence service MI5 as a possible “terrorist threat” in 2020, but the investigation was closed after authorities concluded he posed no threat.

Also Read: Texas rabbi says he, two other hostages escaped synagogue standoff

White House press secretary Jen Psaki said Akram appears not to have set off any alarms in U.S. terrorism databases when he entered the country.

“Our understanding, and obviously we’re still looking into this, is that he was checked against U.S. government databases multiple times prior to entering the country, and the U.S. government did not have any derogatory information about the individual in our systems at the time of entry,” Psaki said.

She added: “We’re certainly looking back … what occurred to learn every possible lesson we can to prevent attacks like this in the future.”

Also Read: Texas Rabbi: Captor grew belligerent late in standoff

The standoff in Colleyville, a city of about 26,000 people 30 miles (48 kilometers) northwest of Dallas, ended after the last of the hostages ran out of the synagogue and an FBI SWAT team rushed in. Akram was killed, but authorities have declined to say who shot him, saying it was still under investigation.

President Joe Biden called the episode an act of terror.

Rabbi Charlie Cytron-Walker, who was among the hostages, told ” CBS Mornings” that he had let Akram into Congregation Beth Israel on Saturday morning because he appeared to need shelter.

Cytron-Walker said the man wasn’t threatening or suspicious at first, but later he heard a gun click as he was praying.

One hostage was released hours later, and the rabbi and two others later escaped after Cytron-Walker threw a chair at the gunman.

During the standoff, Akram could be heard on a Facebook livestream demanding the release of Aafia Siddiqui, a Pakistani neuroscientist who is suspected of having ties to al-Qaida and was convicted of trying to kill U.S. Army officers in Afghanistan. The prison where Siddiqui is serving her sentence is in nearby Fort Worth.

An attorney in Texas who represents Siddiqui said the prisoner had no connection to Akram.

The investigation stretched to England, where over the weekend police announced that two teenagers were in custody in connection with the standoff. The teenagers are Akram’s sons, two U.S. law enforcement officials told The Associated Press. The officials were not authorized to discuss the investigation and spoke on condition of anonymity.

On Tuesday, police in Britain said the teenagers had been released without charge. They were detained in Manchester, about 30 miles from Akram’s hometown of Blackburn.

Akram’s family said he had been “suffering from mental health issues.”

Federal investigators believe Akram bought the handgun used in the hostage-taking in a private sale, according to a person familiar with the matter who spoke on condition of anonymity because the investigation is still going on.