A young mother and her two sons--an infant and a 4-year-old--were killed yesterday morning when fire ignited their mobile home in a Mount Vernon area trailer park.

Firefighters had almost finished their investigation at the scene by late yesterday, and they expected to announce today the cause of the fire, which reduced half the trailer to a few charred wooden studs.

Fairfax County firefighters used a chain saw to slash through the metal-and-wood exterior of the mobile home in the 2900 block of Daisy Street, just off Route 1. They found Jose Lopez Jr., 4, unconscious under some debris near his bedroom door, and 11-week-old Joseluis Lopez in his crib, Deputy Fire Chief Kenneth L. Jones said.

As paramedics tried to resuscitate the children and then rushed them to hospitals, other firefighters found the boys' mother, Maria Gomez, 24, lying in the trailer's living room, near where the heaviest flames and smoke were billowing through the roof and walls, Jones said. Gomez was pronounced dead at the scene.

Gomez's husband, Jose Lopez, 27, owner of El Paso Mexican Restaurant in Springfield, was not home when the fire was discovered about 9:45 a.m. Once he heard about the blaze, Lopez hurried to Inova Mount Vernon Hospital, where one of his sons was taken. Both children died a short time later, Jones said.

Lopez returned to the devastating scene before noon and was comforted by numerous family members and friends. For most of the day, he sat silently on a lawn across the street from his partly incinerated home, staring straight ahead, crying. Investigators said they didn't know where Lopez was when the fire started. He declined to be interviewed.

Gomez, who sometimes waited tables and worked as a cashier at El Paso, was friendly and popular with the staff and customers, family members and others said.

"She was there all day [Saturday], waiting tables, working as a hostess," said James Haburn, a waiter at the restaurant. "She was really nice, really friendly."

Haburn said Gomez often helped on busy nights, even though she had given birth to Joseluis on Oct. 27.

"We'd go there to eat on our days off," Haburn said. "We're all close. He feeds us, he pays us. This is horrible."

The young family sat for a new portrait just last week, said Jacqueline Delgado, Lopez's sister. "The kids were their lives," she said. "They were a happy family. You never saw them fighting."

Delgado said the family moved into Woodley Hills Estates about two years ago. She said that they had come from California and that her brother was born in Mexico.

"They loved dancing and singing," Delgado said. "You always saw her dressed up."

Travis Lewis, 27, said he had been sleeping in the mobile home next door when his mother awoke him with shouts of "fire."

"I went running over there, kicked the door in," Lewis said. "I crawled in as far as I could, but it was too late; the black smoke was too much. I could hear somebody groaning nearby, but I couldn't see anybody."

Lewis said he and family members then used a garden hose "to try to keep the flames contained. It was just too much."

When firefighters arrived, they used an infrared imaging camera to try to find the family through the dense smoke, but it was not until firefighters cut a hole in the rear of the 15-by-60-foot trailer that anyone was located, said firefighter Clark Slaymaker.

Ruth Nesselrodt, president of the neighborhood civic association, had seen all three victims at the park's annual Christmas party Dec. 22. "Everybody was happy and healthy," she said.

Yesterday's fire was the second fatal blaze at a Mount Vernon area trailer park this year. On New Year's morning, Jessica Sprage, 27, died after she suffered burns in a mobile home fire in the 7900 block of Woodpecker Way. That blaze was attributed to an unattended candle, and fire officials saw no link between the two incidents.