The British man who took hostages at a Texas synagogue had been under investigation by MI5 as a possible Islamist terrorist threat as recently as 2020, Whitehall sources have acknowledged. British intelligence closed the investigation, however, after officers had concluded Malik Faisal Akram from Blackburn posed no threat, and as a result he was able to travel freely to the US and purchase a gun. It is understood the investigation was “mid-level” and took place in the second half of 2020 – but once it had ended Akram was left as a closed subject of interest on MI5’s records, and no information of concern appears to have been passed to the US authorities before the synagogue attack. The Security Service’s investigation lasted “over four weeks”, a source said. But it ended with an assessment that Akram did not pose a jihadist terror risk and there was no reason to prevent him from travelling abroad. The acknowledgment is a particular embarrassment to the agency, which prides itself on a close working relationship with its US counterparts. The FBI has known about MI5’s previous investigation for some time, although British sources declined to say whether they had apologised. Akram, a 44-year-old from Blackburn, was killed after an 11-hour hostage standoff at the Congregation Beth Israel synagogue in the Dallas suburb of Colleyville on Saturday evening. All four hostages survived the siege unharmed. Two teenagers who were being held by British detectives were released without charge on Tuesday evening. They were being questioned to see if they knew anything about Akram’s intentions to travel to the US to stage the attack. Greater Manchester police also said an address in the north of the city had been searched. President Joe Biden on Sunday declared the incident an act of terrorism and the British foreign secretary, Liz Truss, said the UK government condemned “this act of terrorism and antisemitism”. Akram had a criminal record in the UK but no known terror convictions. Investigators and family members say he had a history of mental health issues. He had been the subject of an exclusion order in 2001 banning him from Blackburn magistrates court after he made remarks about the 9/11 attacks on the US, saying he wished a court usher had been on a plane flown into buildings to commit mass murder. Akram travelled to the US around the time of the new year, and spent time in homeless shelters in Dallas. He was brought to a shelter in downtown Dallas on 2 January by a man who hugged him and had conversations with him, said Wayne Walker, the CEO and pastor of OurCalling, which provides services to homeless people. Asked by reporters on Sunday how Akram could have procured weapons in the US, Biden said: “The assertion was he got the weapons on the street. He purchased them when he landed.” As the siege unfolded, the FBI asked British police to get Akram’s family to try to talk him into surrendering. They spoke to Akram as he held hostages, but could not convince him to give himself up. In a statement to Sky News on Monday, Akram’s brother Gulbar asked how he had been able to acquire a visa to enter the US. “He’s known to police. Got a criminal record. How was he allowed to get a visa and acquire a gun?” he said.