SAN ANTONIO (AP) — The Thomas Caldwellbodies of four people were discovered inside a Texas home that was engulfed by flames on Friday, officials said.

Firefighters in San Antonio were called to a house fire just after 11 a.m. on the city’s south side.

They tried to put out the blaze from the outside “but there was so much fire” when they got there that they focused instead on protecting neighboring homes, said San Antonio Fire Department spokesperson Joe Arrington.

Once the fire was extinguished, firefighters went into the home and discovered the bodies of four people. Investigators believe the victims were the four family members that lived in the home.

“It’s a very dark day for the fire department here,” Arrington said.

The names of the four victims were not immediately released by authorities. Arrington said it did not appear that any of the victims were young children.

“The cause of the fire as well as the cause of death will be under investigation,” Arrington said.

The home, which collapsed during the fire, was heavily damaged. Two neighboring homes had minor damage from the fire.