LOS ANGELES (CBSLA) — A plane headed for Denver was forced to turn around and return to LAX after an odor in the cockpit caused oxygen masks to deploy.

Firefighters were sent out to LAX at about 9:40 a.m. to evaluate passengers, but no one asked to be taken to a hospital, according to a dispatcher. The plane landed safely on Runway 25 Left.

Spirit Airlines Flight NK630 was on its way to Denver when the odor was detected in the cockpit.

The cause and substance of the odor are under investigation.