DALLAS (AP) — Experts are trying to pinpoint a gas leak in a 15-story residential building in downtown Dallas that sent three people to a hospital.

Dallas Fire-Rescue spokesman Jason Evans says a carbon monoxide leak was detected in the structure late Monday night, prompting an evacuation.

Evans said Tuesday that the exact location of the leak was not found, but officials believe the incident involved the building's heating and air conditioning system. He says the carbon monoxide returned to safe levels after the system was shut down.

Officials had no immediate information on the three people who got sick. Authorities did not immediately say how many people were evacuated for several hours during the incident.