MONROVIA (CBSLA.com) — A 17-year-old student suspected of intentionally setting fire to Monrovia High School has been arrested, police said Monday.

According to the Monrovia Fire Department, the third-alarm blaze erupted shortly after 12:30 a.m., located in the 800 block of West Colorado Boulevard.

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Authorities arrived on scene and located flames and heavy smoke pouring out of the third floor of the building.

Investigators say the fire erupted within the counseling office on campus, where an undetermined but “severe” amount of damage was found.

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Witnesses said they saw a boy throw something through a window to start the fire.

No injuries were reported.

Classes were in session Monday despite the blaze, officials said.

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The cause of the fire remains under investigation.