Three people died and nearly 20 first responders were quarantined Wednesday after exposure to an unidentified substance at a New Mexico home, authorities said.

New Mexico State Police said officers responded around 11 a.m. to a residence in Mountainair, New Mexico, to assist the Torrance County Sheriff’s Office with what was initially believed to be a suspected overdose involving an unknown substance.

Authorities said four people were found unresponsive inside the residence. Three of them were pronounced dead, according to a police statement.

Officials said 18 first responders were exposed to the substance and later began experiencing symptoms including nausea and dizziness.

MAINE STUDENT PASSES OUT, 11 TAKEN TO HOSPITAL AS CAUSE OF ILLNESSES REMAINS UNKNOWN

The fourth occupant of the residence and the exposed first responders were transported to the University of New Mexico Hospital, where they are being quarantined, evaluated and monitored.

Two first responders remain in serious condition, police said.

Albuquerque Fire Rescue hazmat teams are working to identify the substance involved.

Investigators said they believe the substance may spread through contact but do not believe it is airborne.

Authorities said there is no threat to the public and that a secure perimeter has been established around the residence.

This is a developing story; check back for updates.