Elevated levels of carbon monoxide inside a Selden medical office Tuesday morning sent three people to hospitals for treatment, authorities said.

About three dozen people were inside the Boyle Road building when an 11:17 a.m. 911 call came in reporting carbon monoxide detectors going off in the medical office, Suffolk police said.

The Selden Fire Department responded and “quite a number” of patients were evaluated for carbon monoxide exposure, police said.

Three people were transported to hospitals for treatment of “non-life threatening levels” of carbon monoxide exposure, police said.

Photos taken in the parking lot outside the medical office showed emergency personnel evaluating people, including at least one person on a stretcher.

Authorities said there were elevated carbon-monoxide levels in the building, causing the exposures.

First responders had not identified the source as of Tuesday afternoon and were still trying to determine what had caused the elevated levels, officials said.

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Police also said carbon monoxide levels in the building had “returned to normal” by 12:30 p.m.