Authorities are investigating the cause of elevated carbon monoxide levels in three Ancell Street townhouses that left three children hospitalized Monday morning. 

Fire officials received a call from one of the homeowners complaining of headaches and nausea about 6:56 a.m. As paramedics arrived, a neighbors carbon monoxide detector went off, confirming what authorities already suspected was a case of carbon monoxide poisoning, said Alexandria Fire Department spokesman Bob Rodriguez.

The three homes affected share walls, he said. Three children were transported to the hospital from the home where the emergency call originated. The block units other residents declined medical treatment, Rodriguez said.

The childrens condition is not thought to be serious, officials said. 

Authorities have not determined a cause yet, but Rodriguez said any homeowner with fuel-fired appliances should install a carbon monoxide detector. Additionally, homeowners should have their furnaces serviced annually by a licensed contractor, he said.