Two 6-year-old children died yesterday after they were found, one atop the other, by the front door of a locked and burning apartment in Reston, Fairfax County fire officials said.

Jermaine James, who lived in the apartment, and Amanda L. Croson, a friend from the neighborhood, were pronounced dead at Reston Hospital Center of injuries received in the 10:30 a.m. fire, officials said.

Investigators were still trying late yesterday to determine the cause of the fire, which gutted the third-floor condominium apartment at 11633 Stoneview Sq. in the Shadowood subdivision, causing an estimated $150,000 in damages. Criminal charges were possible, according to sources.

The sources said that Jermaine's 8-year-old sister, Tina, was baby-sitting with him yesterday morning and that Amanda had stopped by to play. After the fire began, Tina ran from the apartment and the door locked behind her, they said.

"She was yelling and crying," said Melanie Beale, 16, a neighbor who said she saw Tina running outside. "She said her brother was in the house."

A maintenance worker with a pass key unsuccessfully tried to open the front door of the apartment, and two other workers made rescue attempts from the other side of the building, but the fire was already roaring, according to officials with the Shadowood Condominium Association.

"It was totally engulfed in flames," said Sharon Balz, 46, another neighbor. "There were flames shooting out of the windows."

Firefighters arrived within minutes after receiving the 10:31 a.m. call and broke down the apartment's front door, county Fire and Rescue Department spokeswoman Pam Weiger said. "Right inside, on top of one another, were the two kids," she said. Firefighters administered cardiopulmonary resuscitation.

Officials at the condominium association office said that the James family has been renting the unit since December.

Sandra James, the mother of Tina and Jermaine, was working at a local motel when the fire occurred, one source said. Neighbors said the father, Larry James, is employed as a painter.

Larry James went to the charred apartment yesterday afternoon and bolted up the stairs. Minutes later, crumpled over, he was escorted away by another man in painter's overalls.

Condominium officials said that Amanda Croson lived with her mother, also named Amanda, at 11621 Stoneview Sq., and that Croson has owned the unit since 1979.

Arrangements were being made yesterday for a counselor to help the maintenance workers and another employe distraught over the children's deaths, said Lou Goddard, an official on the condomimium's board of directors. A fire official said a psychologist will be available to help the firefighters deal with the deaths, the fourth and fifth fire fatalities in the county this year.

Yesterday's blaze was not the first major fire on Stoneview Square. In February 1986, firefighters rescued four screaming children who had been left unattended in an apartment that caught fire at 11655 Stoneview Sq. All four children survived.

Outside the apartment where Jermaine James had lived, Maggie Thompson, 6, was coping yesterday with the loss of Amanda Croson, her best friend. "Well, you see, we always used to play together. When we played, we had more fun. The funnest was when we played with Play-Doh," said Maggie, her mother at her side. "But now that this happened . . . , I don't think I have any best friend."