HUDSON (CBS) — Hazmat crews responded to a large apartment complex in Hudson Thursday after an unknown odor was reported.

The first two firefighters who entered a common area of the 118-unit building were overcome.

The firefighters were taken to an area hospital, but returned to the scene a short time later.

“They did inhale, which gave them some respiratory problems. Felt like they were having a hard time breathing, sore throat,” said Fire Chief John Blood.

Fire and Hazmat crews outside an apartment complex in Hudson Thursday afternoon. (WBZ-TV)

The State Fire Marshal’s office said the odor prompted a large hazmat response at the building on Central Street.

Crews tested air levels in the building, but found nothing to explain what sickened the two firefighters.

“It could have been something that someone inadvertently or accidentally released,” said Fire Marshal Peter Ostroskey.

Hazmat crews test the air inside a Hudson apartment. (WBZ-TV)

Firefighters opened as many windows as possible throughout the building to ventilate.

Residents have been let back into the building.

The cause of the incident remains a mystery.