Monday marks the 31st anniversary of the Alton bus crash.

The tragedy, which killed 21 students, is the worst school bus crash in Texas history.

The crash happened on Sept. 21, 1989, when a truck hit a school bus — sending the bus into a pit that had filled with water.

After the crash, new safety standards were imposed on schools buses in an effort to protect students.

Watch the video for the full story.