Police are investigating an active shooter situation reported at Timberview High School in Arlington, Texas, the school district said.

An unknown suspect apparently shot multiple people before fleeing the scene, according to an internal police briefing. The number of victims was not immediately clear.

The school is on lockdown, the Mansfield Independent School District said. The scene is secure, according to the internal briefing.

Arlington police said they are conducting a “methodical search.” ATF officials are at the scene.

Arlington is located between Fort Worth and Dallas.

This is a developing story. Please check back for updates.