BEDFORD (CBS) — Two people were overcome by carbon monoxide at an apartment building in Bedford Sunday and had to be taken to a local hospital.

Bedford Police say a call came in at 5 p.m. for an apparent CO alarm activation at the apartment building on Albion Road.

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Upon arrival, the Bedford Fire Department reported high levels of carbon monoxide inside and ordered a complete evacuation of the building’s 20 apartments.

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Two residents had to be taken by ambulance to Lahey Hospital in Burlington, but they are expected to recover.

After an investigation, all residents were accounted for, and it was determined that the problem originated in the building’s boiler. Building management arrived on scene to affect repairs and to assist residents. Residents were allowed back inside around 8 p.m.

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“Carbon monoxide is a silent killer, and we are fortunate that all residents made it out safely,” Bedford Fire Chief David Grunes said.