The father of the 29-year-old who killed 49 people at an Orlando, Florida, nightclub in 2016 was an FBI informant.
He came under scrutiny himself after investigators found receipts for money transfers to Turkey and Afghanistan in the wake of the mass shooting, recently filed court documents show.
The revelation came in documents filed by defence lawyers for the shooter's wife, Noor Salman, who is on trial in Orlando on allegations that she aided and abetted her husband's attack and obstructed law enforcement's investigation into it.
Prosecutors argue that Salman essentially admitted to the FBI that she knew what her husband was about to do. Defence lawyers argue that she herself was a victim of her husband's abuse and infidelity and that false statements she made to the FBI came during a coercive interview.
The attack by her husband, Omar Mateen, is one of the deadliest mass shootings on US soil. Omar Mateen said before police killed him that it was done in the name of the Islamic State terrorist group.
Salman's trial has been under way for weeks, but defence lawyers argued that they were not told about the father's FBI work until Sunday. That, they argued, is grounds to dismiss the charges against their client, or at least declare a mistrial.
"It is apparent from the Government's belated disclosure that Ms Salman has been defending a case without a complete set of facts and evidence that the Government was required to disclose," defence lawyers wrote.
Mateen's father, Seddique Mateen, was an FBI informant at various points between January 2005 and June 2016, court documents say.
Salman's defence lawyers wrote that Seddique Mateen also had sent money to Turkey for a "substantial period and just one week before the attack", and that in November 2012 an anonymous tip alleged he was trying to raise between US$50,000 and US$100,000 ($68,000-$137,000) to help fund an attack against the Government of Pakistan.
Prosecutors acknowledged the relationship between the FBI and Seddique Mateen in an email quoted in the defence lawyers' filing. They also acknowledged that investigators searching Seddique Mateen's home after the shooting found receipts for money transfers to Turkey and Afghanistan, which were dated between March 16, 2016 and June 5, 2016.
Based on those receipts, the FBI opened an investigation into Seddique Mateen, prosecutors wrote.
Efforts to reach Seddique Mateen today were unsuccessful. The FBI declined to comment.
After the massacre, Seddique Mateen had said in interviews that he saw his son the day before the shooting and that nothing seemed amiss.
"He was well behaved. His appearance was perfect," he said. "I didn't see any sign of worrying or being upset or nervous."
Salman's lawyers argued in court filings that if they had known of Seddique Mateen's work for the bureau, they might have explored a theory that Omar Mateen "conspired with his father, rather than Noor Salman, to commit the acts".
Defence lawyers argued that they also might have explored whether the FBI's interviews with Salman were an attempt at "evading the negligence they exercised with their own informant", and whether their "unwavering focus on Noor Salman, rather than Seddique Mateen, could have been designed to find a culprit other than the father".
The FBI has previously come under criticism for investigating Omar Mateen for 10 months starting in 2013 and ultimately concluding he was not a threat.
Defence lawyers frequently ask a judge to throw out a case before it goes to a jury, and such requests are seldom granted. Prosecutors' failure to turn over relevant information, though, is considered a serious transgression, and lawyers for Salman argued that this was not the Government's first offence in the case.
Prosecutors, they said, also failed to initially notify them that forensic evidence contradicted Salman's assertion that she had visited the nightclub before the shooting.
Closing arguments in Salman's trial are expected this week.