LYNN – Police arrested a Lynn man Wednesday believing he fired a gun outside Classical High School on Friday, resulting in a lockdown.But when Many Keo, 19, got to court, authorities were forced to dismiss all the charges against him after his alibi checked out.Keo was arrested shortly after midnight Wednesday morning and charged with discharging a firearm within 500 feet of a building and illegal possession of a firearm and ammunition.According to police reports, two eyewitnesses to Friday afternoon’s shooting on O’Callaghan Way pointed out Keo as the shooter in a photo line-up.As Keo was being arraigned, prosecutors sought a $15,000 bail. But at some point during the arraignment, Keo’s alibi was called into question.According to a police source, Keo claimed he was at work when the shooting occurred.Investigators quickly checked into the claim and verified through video surveillance that Keo was at his workplace when shots rang out at 2:30 p.m. Friday.”Subsequent to the identification of Mr. Keo and his arrest, Lynn police continued to investigate the case and verified he had an alibi at the time of the shooting,” Essex County District Attorney spokeswoman Karen Dawley said. “As soon as we learned of this information, we notified the court and entered a nolle pros to cease the prosecution of the case in the interest of justice.”Keo, a Pinkham Place resident, was released from custody after Lynn District Court Judge Stacey Fortes-White had no choice but to dismiss all of the charges against him.Keo’s arrest marked the third police have made since the shooting terrified hundreds of people and sent three schools into lockdown on Friday.Ngoc My Chau, 20, of 17 Whiting St., Lynn, was arrested Monday and charged with discharging a firearm within 500 feet of a building and illegal weapon possession. Eyewitnesses to the shooting wrote down the license plate number of the shooter, which was linked back to Chau’s older brother.In a tape-recorded interview, Chau admitted he was driving the vehicle when bullets began flying, but denied being with anyone who had a gun, according to court documents.Steven V. My, 18, of 19 Chase St., Lynn, was arrested for trespassing and disorderly conduct after he was seen running through backyards when shots rang out. He is a Classical High student, according to police.Police say the investigation remains ongoing.