A 28-year-old Woodbridge mother and her young son burned to death Friday evening when their mobil home -- which had no smoke detector -- was suddenly engulfed in flames. Prince William County firefighters said yesterday.

Twelve hours later, before dawn at a house trailer park, seven miles to the south in Dumfries, a second mobile home was destroyed by fire, said officials. But a smoke alarm roused the Monroe Jeffers family, enabling them to escape unharmed.

County fire officials said yesterday that mobile homes clustered along the Rte. 1 corridor popular with military families and young couples unable to afford more expensive suburban Virginia housing -- are a cause of special concern.

"It's like 'poof' and they're gone," said Lt. Roger Cordani of the Prince William fire department.

Cordani and other fire officials -- stressing the effectiveness of smoke alarms -- said the trailers' construction, usually of thin aluminum, tin, paneled and compressed wood and polyurethane, make them susceptible to rapid burning.

"I looked out my window and the fire had just started," said Mike Perry, a neighbor of the Jefferses and their 35-foot trailer at Watson's Trailer Park on South Main Street in Dumfries. "It only took me a minute to get outside, but half of the trailer was already burning by then."

Fire officials said there have been several house trailer fires in the county this year, including at least one fatal blaze prior to Friday night's incident.

Shortly after 4 p.m. Friday, according to fire investigators, Sandra Brooks and two-year-old son Mathew had gone to their bedroom for an afternoon nap at their two-family house trailer in the Marumsco Mobile Home park on Jefferson Davis Highway.

"Miriam Stoffard, she's 70 and has had a heart attack, saw the fire and woke up the guy living there. It was just so hot that we couldn't get near it. We heard her (Brooks) scream once then the flames just billowed out," said Debbie Burton, 22.

Only a shell remained of the Brooks trailer yesterday as it stood beneath a charred weeping willow on lot 426 at the park.

"We were able to get a couple of steps inside a door, but no further," said 20-year-old Ralph Martin, the Brooks' next-door neighbor who suffered minor burns in the abortive rescue attempt. "She was hollering and you could hear the baby choking. It was awful."

Brooks and her husband were separated, fire officials said.

A report filed by fire investigator Ray Schley said that both victims were found in the northside bedroom of the trailer, but the mother "was only two steps from the door and the child was still on the bed."

Prince William volunteer Capt. John McGovern said "the aluminum skins of the trailers just melt, the fires are so intense. It concerns us. We're always concerned. If we could, we'd probably buy smoke alarms for people."

There are nine separate trailer parks along the Richmond Highway corridor in Fairfax County, according to Lt. Bill Schum of the Penndaw fire station. "The trailers are usually fully engulged when we arrive. Often the smoke won't wake a person up in time for them to escape," Schum said.

Fairfax County passed a zoning ordinance change in 1970 that placed stricter rules on trailer parks, including requirements for paved streets, fire hydrants and curbs, Schum said. But the changes were not retroactive and did not affect parks built as early as the 1940s near Fort Belvoir.

The county is threatening to condemn one such park, Schum said, but he declined to release the name because of pending litigation.

Sandra Brooks and her son had lived in Marumsco for only a month before the fatal fire, according to the park's manager. Monroe Jeffers had just bought his $20,000 home a month before the fire, Perry said.

"They lost a chihuahua puppy in the fire. I never saw a trailer fire before in my life. And it was brand new and beautiful with a sunken den and everything. Now it's just a mess," Perry added.

Fire officials said the cause of both fires is under investigation.