Police said that on Monday, February 23, that they arrested a youth under age 18 on charges of first-degree threatening and first-degree falsely reporting an incident at Newtown High School.

Police allege that the boy wrote a suspicious note which was discovered by a school staff member, resulting  in the school temporarily entering a “lock-in open” security mode.

Police Lieutenant Richard Robinson said both charges filed against the boy are Class D felonies.

Police did not disclose the boy’s identity because he is under age 18.

The youth was booked on the charges and then released into the custody of parents for a March 9 appearance in Danbury Juvenile Court.

Lt Robinson declined to disclose the contents of the note which was written on a piece of paper.

The discovery of  the suspicious note at about 8:15 am prompted the school to enter a “lock-in open” security status, wherein teachers and students continue with the school routine and are allowed to leave the school, but visitors may not enter the building. The security status was later lifted after a police investigation into the situation.

Police and school officials created such security modes as an aspect of emergency planning following the December 2012 shootings at Sandy Hook Elementary School.

School staff members and students were immediately notified of the security situation, but normal business within the school continued, Lt Robinson said.

The school building was checked for suspicious activity and for suspicious items by the school’s emergency response team, the school security staff, and by town police, Lt Robinson said.

“Nothing suspicious was located,” he said.

“At no time did the Newtown Police Department believe there was any threat to students or staff,” Lt Robinson said.

“It was determined that the threat to the school was an isolated incident, was not credible, and that no one was in danger,” according to police.

The police detective unit is probing the incident, with School Resource Officer Liam Seabrook serving as the primary investigator.

(This story was updated at 4:15 pm 2/23/15 to include additional details from the Newtown Police Department.)