LOS ANGELES (AP) — Four people have been sickened by an apparent release of carbon dioxide at a Los Angeles International Airport baggage area.

Los Angeles Fire Department spokesperson Brian Humphrey says in a statement that one person is in grave condition and three are in mild distress.

There is no information on their identities. The statement says the source of the gas in the Terminal 8 baggage area early Monday is “unspecified.”

The airport tweets that Terminal 8 is being cleared of passengers, who are being sent to Terminal 7 for screening. United Airlines and United Express operate at Terminal 8.