In a 24 January interview, Mr Williams admitted to selling the weapon to a man with a "British accent" in south Dallas.
This came after Mr Akram posed as homeless in order to enter the Colleyville-based shul on 15 January, during a morning service, then brandished the recently purchased gun.
Mr Akram released one hostage after six hours, while Rabbi Charlie Cytron-Walker and two other worshippers escaped several hours later after the former threw an item of furniture toward the attacker.
Throughout the terrifying standoff, the 44-year-old British national demanded the release of Pakistani scientist Aafia Siddiqui, currently serving an 86-year prison sentence in a nearby federal prison after a 2010 conviction for plotting to kill American soldiers in an Afghan police station.
Earlier this year, Ms Siddiqui issued a statement via her attorney in which she distanced herself from the January attack.
