A University of Wisconsin-Eau Claire student from Stillwater killed in an early-morning apartment fire Monday died of carbon monoxide poisoning from smoke inhalation, an autopsy has found.

Ross A. Livermore was pronounced dead Monday at Mayo Clinic Health System in Eau Claire. He lived with three Stillwater area friends in an off-campus Eau Claire apartment that went up in flames about 5:45 a.m.

One of the roommates, Jacob Clarkson, was badly burned and flown to Hennepin County Medical Center, where he died Tuesday afternoon. His official cause of death has not been determined.

The other two roommates, Garret Isakson and Casey Malan, escaped the burning building with minor injuries. All four were 21 years old and graduated from Stillwater High School in 2009.

The cause of the fire was still being investigated, said Scott Burkart, deputy chief of the Eau Claire Fire Department, and it may take two weeks or more to conclude.

Officials said there were smoke detectors in the apartment.