Nadeem Ilyas Patel, 26, from east London was charged with the possession of a firearm with intent to cause fear of violence in May last year.
He was remanded in custody along with Umar Ahmed Haque, 25, Muhammed Abid, 27, and Abuthater Mamun, 19, who were all on trial at the Old Bailey court in London.
The jury found Patel not guilty of conspiring with Haque to get hold of a gun for a terror plot. He had admitted before the trial to the illegal possession of a blank-firing gun.
The jury was told that Haque had taken limited steps towards fulfilling a plan to attack a target in London and sought to radicalise children in a small mosque in the city.
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Police believe that he came into contact with more than 100 children, some of whom have since received de-radicalisation support.
Haque was arrested two weeks before the London Bridge and Borough Market terror attack in London in June 2017.
Haque, a schoolteacher, may have radicalised about 110 children, 35 of whom are receiving long-term safeguarding support, said Commander Dean Haydon, head of counter-terrorism at Scotland Yard.
"Haque paralysed those children through fear showing them really distressing videos. We faced a wall of silence as a result. It was very challenging to fully understand what was going on in those classrooms," he said.
In hours of secret recordings from March 2017, played to the trial, Haque eulogised Khalid Masood, the man who had just carried out the first of 2017's terrorism attacks on the UK Parliament.
He told his two accomplices that he was training schoolchildren to be a "death squad" and he aspired to attack Big Ben, Heathrow Airport and other locations in London.
Haque became a priority for the UK's security services after he was stopped from leaving the UK in April 2016 because of suspicions he was heading to Syria.
Police searches of Haque's home found a notebook detailing his beliefs in martyrdom and an attack to-do list including to purchase weapons.
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