Crayton and Casey Poplin were rescued by passing motorists as their mother was inside a house on Highway 78 in rural Collin County.

The boys suffered 2nd and 3rd degree burns.

Arthur Covarrubio and his co-workers spotted smoke coming from the windows of a parked Honda Passport sports utility vehicle as they were driving down Highway 78.

They ran up to put out the fire and were stunned at what they found. "There was a kid in there, and the whole inside of the car was burning," Covarrubio said.

One man grabbed the boy, then Covarrubio spotted his twin -- his arms reaching out for help. "I opened the door and there were just flames crawling up the kid, the front of the kid," he said.

Patricia Aaron, the boys' mother, came rushing outside -- frantic. Her three-year-olds, Casey and Crayton, had been badly burned.

"Their shoes were melted they were burnt so bad," Covarrubio remembered.

The children and their rescuers then had to wait until emergency personnel arrived at the rural home about 10 miles northeast of McKinney. "All of us were in tears because we can't do anything for these kids," Covarrubio said.

Rescue crews airlifted the children to the burn center at Parkland Memorial Hospital in Dallas, where they were admitted in crtical condition.

The 35-year-old mother told authorities she had stepped inside to check with her boss on part-time work. She said she constantly checked on the twins through the window.

But she didn't see what fire investigators later found in the back seat -- two cigarette lighters which may have ignited the blaze.

"It's the only heat source we can find back there at this time," said Collin County Fire Marshal Steve Diffebaugh.

Diffebaugh praised the men who stopped to help the burning youngsters. "These gentlemen here are heroes," he said. "They got them kids out of the car."

Aaron's boss wants people to know she is a good mom who made a tragic choice by leaving them alone for a few minutes just a few feet away.

Casey and Clayton were both still reported in critical condition late Thursday afternoon.

Collin County officials said they were unsure whether any charges will be filed in the case.

The incident happened one day after an infant was found dead strapped in a car seat in a Dallas parking lot. His mother told police she thought she had taken the child to daycare.