Questions are mounting over how Faisal Akram, who also had previous criminal convictions, was able to obtain a visa and travel to his target.
The 44-year-old, originally from Blackburn, was shot dead by law enforcement after taking hostages at the Congregation Beth Israel synagogue in Colleyville, Texas, on Saturday.
It is not known when he first came onto MI5’s radar, and he is not believed to have been considered an imminent security threat.
The BBC reported that he was most recently the the subject of an investigation in late 2020, but the probe was ended - leaving Akram in a pool of over 40,000 closed subjects of interest.
Akram was also known to local police in Lancashire for previous criminal offences, and in 2001 had been banned from Blackburn Magistrates’ Court after ranting about the 9/11 terror attacks.
Speaking to Sky News, the attacker’s brother Gulbar Akram questioned how the incident had been allowed to unfold.
“He's known to police, got a criminal record,” he said. “How was he allowed to get a visa and acquire a gun?”
The gunman had links to the Blackburn, Burnage and Manchester areas, and flew into New York’s John F Kennedy airport around two weeks ago.
President Joe Biden said he spent at least one night in a homeless shelter and purchased the gun used in the attack “on the street”.
He branded the incident “an act of terror” and said Akram made “antisemitic and anti-Israeli comments”.
The FBI, with support from British counterterror police, are investigating why Akram targeted the synagogue and the motivations behind the hostage-taking.
Counter Terror Policing North West announced that two teenagers had been arrested in Manchester on Sunday night, in relation to the attack, but gave no further details.
Priti Patel said she had spoken to her US counterpart, Alejandro Mayorkas, and offered “the full support” of the UK police and security services in the investigation.
One hostage was freed after around six hours, and three others managed to flee out of an exit before the FBI stormed the building and shot Akram dead.
Rabbi Charlie Cytron-Walker said he had invited Akram into the synagogue and made him a cup of tea before the start of scheduled prayers, unaware of his intentions.
He said that during the service he “heard a click” and turned to see the man had produced a gun, starting a 10-hour standoff.
Rabbi Cytron-Walker said the situation worsened, and the gunman “didn’t sound good” as his demands for the release of a Pakistani woman jailed in the US were not met.
“We were terrified,” he told the CBS news channel. “When I saw an opportunity where he wasn't in a good position … I threw a chair at the gunman and I headed for the door. And all three of us were able to get out without even a shot being fired.”
Part of the incident was livestreamed on Facebook, after being caught on the synagogue’s routine broadcast of its Shabbat morning service, but was taken offline by the social media giant.
Akram could be heard talking to what appeared to be an FBI negotiator and family members on the phone.
“Don’t f***ing cry over me, have a party,” he said at one point. “I’ve got hostages and I’m surrounded and I’m going to die, OK?”
He demanded the release of Pakistani neuroscientist Aafia Siddiqui, who was convicted of trying to kill US army officers in Afghanistan, and is in prison in Texas.
Her case has become a cause celebre among terrorist groups including al-Qaeda and Isis, and her release has been among the demands listed by previous hostage-takers in Pakistan, Afghanistan, Algeria and elsewhere.
Speaking to reporters after the synagogue standoff ended, FBI special agent in charge Matt DeSarno said they believed Akram was “singularly focused on one issue and it was not specifically related to the Jewish community”, and added they will continue to “work to find motive”.
Police have not confirmed the details of any previous offending by Akram, but relatives said he had a criminal record.
He was banned from Blackburn Magistrates’ Court in 2001 after threatening and abusing staff on several occasions.
A letter from the court, published by the Lancashire Telegraph, recounted an incident on 12 September 2001, the day after the 9/11 attacks.
“In a clear reference to the the terrorist attack on New York the previous day you said on more than one occasion to one of my court ushers 'you should have been on the ******* plane’,” the letter said.
“This caused a great deal of distress to an individual who was simply doing his job and should not be subjected to your foul abuse.
“With immediate effect it has been decided that in order to protect and ensure the health and safety of staff you should be excluded from and prohibited from entering the court building at all times, other than when due to appear in court.”
In a statement posted on Facebook, the attacker’s brother Gulbar Akram wrote: “We would like to say that we as a family do not condone any of his actions and would like to sincerely apologise wholeheartedly to all the victims involved in the unfortunate incident.”
Mr Akram suggested he had been present for negotiations near the synagogue, and said: “Although my brother was suffering from mental health issues we were confident that he would not harm the hostages … there was nothing we could have said to him or done that would have convinced him to surrender.”
The statement was deleted from the Blackburn Muslim Community Facebook page on Monday morning, alongside a tribute that had sparked a backlash.
“May the Almighty forgive all his sins and bless him with the highest ranks of paradise,” it had read.
“There are many stories circulating in the local community so please avoid taking part in the sin of backbiting.”
In a new statement, the administrators of the Blackburn Muslim Community page said the post had “utilised a standard template with generic wording” for death announcements, and had sparked a wave of threats.
“After learning about the full circumstances surrounding his death, the post was removed,” a spokesperson added.
“We apologise for any upset or offence caused to those directly and indirectly affected by the incident especially the Jewish community in Texas. This was unintentional and our thoughts are with them all. We totally condemn any threats or attacks on innocent people.”