MILL VALLEY (CBS SF) – Authorities on Wednesday announced the arrest of a suspect in connection with a Monday incident at Tamalpais High School involving a reported armed gunman near the school campus.

Sheriff’s officials initially declined to provide additional details regarding the arrest but said they wanted to inform parents of the development prior to the start of class Wednesday.

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During a late morning press conference, the Marin County Sheriff’s Office announced that a former Tamalpais High student, 21-year-old Michael Kessler from Fairfield, was the person responsible for the Monday incident.

A report about a person wearing a black mask and possibly carrying an AK-47 type rifle behind the high school in Mill Valley caused a lockdown and shelter in place Monday afternoon, police said.

Officers responded around 1:50 p.m. to a report about a possible gunman wearing black clothes on Almonte Boulevard. Another person reported seeing a suspect wearing a mask and similar clothing near parked vehicles but a gun was not seen, police said.

Police and sheriff’s deputies checked the area for a gunman and a white car, and authorities issued a shelter-in-place order that included a lockdown of the high school and Mill Valley Middle School.

On Wednesday, Marin County Sheriff’s Office spokesperson Brenton Schneider said that a deputy spotted a vehicle in the nearby Good Earth parking lot and ran the license plate, which connected it to a local car rental company.

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On Tuesday, a witness reported running into a man in the Good Earth parking lot during the Monday incident who pulled a ski mask down over his face an asked about the police response in the area. Investigators were able to identify Kessler at the suspect thanks to the witness description and information from the rental car company.

Late Tuesday night after interviews with Kessler and other witnesses, detectives were able to recover an Airsoft replica AK-47 that had electrical tape placed over the orange tip at the end of the gun that had been discarded in bushes near the residence of relatives of Kessler’s in Mill Valley.

Kessler admitted to authorities that he had used the Airsoft gun and a mask while shooting a Snapchat video near Tamalpais High School. He also said that he was aware and following authorities’ social media posts regarding the police response to the report of an armed man.

Kessler has been arrested on charges of possessing a weapon near school grounds, destroying or concealing evidence, brandishing a replica firearm, unlawfully displaying a replica firearm and changing the markings on an replica firearm to make it look real.

“We have no reason to believe that Kessler, who was a former student at Tamalpais High School, had any intention of performing an act of violence on the school,” said Schneider. “Kessler was aware of the fear that his actions caused the public and Tam High School students and was remorseful.”

Schneider also praised witnesses for coming forward and providing information that helped investigators make an arrest.

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