HOUSTON -- Burglar bars are the focus of an investigation into a fire that killed three children, KPRC Local 2 reports.

A set of 3-year-old twin girls and a 4-year-old boy died in the blaze at the St. Joseph Condominiums on Park Place Boulevard at Hartford Street at about 3 p.m. Tuesday.

Firefighters had a difficult time reaching them because of security bars on the windows and a steel door.

Rescuers used equipment to cut through walls and the door.

"The burglar bars are cut into the masonry. It was a huge issue for them to get in. Initially, we were not told there were children inside. I've heard conflicting stories," HFD District Chief Tommy Dowdy said. "Once they were told, to me, they made a Herculean effort to get into the apartment."

Officials said language was a barrier in determining if there were people inside.

A witness said the children's grandmother went to the store and left a 15-year-old in charge. The witness said the teenager went outside to play and locked the children inside. The teen was not able to get back inside the condo when the fire broke out.

Fire officials said people with burglar bars need to be careful.

"If you have burglar bars you want to make sure that you have a designed burglar bar that has a quick release mechanism so that your family will know how to get out," said Rick Flanagan of the Houston Fire Department. "You have got to have a rehearsal process with your family on how to get out in the event of a fire."

Children's Protective Services officials said they are not yet investigating the case.

The cause of the fire is under investigation.

The identities of the children were not released.