COLUMBIA, Mo. (AP) — Three of four people injured in a hydrogen gas explosion inside a University of Missouri biochemistry lab have been released from the hospital.

The university says the fourth person remained hospitalized in good condition Monday night.

The blast happened after 2 p.m. on the third floor of Schweitzer Hall on the Columbia campus.

The university says the explosion is believed to have been caused by a "spontaneous combustion of gases" being used in a research experiment.

The names of the injured weren't immediately released. A campus spokesman says he doesn't know if they were professors, students or graduate researchers.

One campus worker says she heard the explosion several blocks away. The building where the blast occurred had blown-out windows and melted window blinds.