LAS MILPAS – Fire investigators determined the cause of at least one of the three house fires which sparked overnight.

Two were deadly; with another cold night underway, Rio Grande Valley fire authorities want residents to safely stay warm.

The number of calls fire officials receive goes up as temperatures drop.

Tuesday night was the perfect example when Pharr, San Juan and Alamo officials were all dispatched to different fires.

The Pharr fire chief told CHANNEL 5 NEWS his crew were able to put out the fire in about an hour, but one person didn’t make it. The cause is still under investigation.

The Alamo fire chief said the blaze in Alamo was caused by a furnace that hadn’t been used in a long time. No fatalities were reported.

Officials are still investigating the cause of the fire in San Juan, which killed one person.

A resident living near the fire in Las Milpas says all the overnight fires are an indication of the dangers when the temperatures outside drop.

Watch the video above for the full story.