(Update: Investigation continues of plane crash that left six dead in Maryland)

Correction: This story has been updted to correct the description of where Marie Gemmell had worked before going on maternity leave. She had worked for First Potomac Realty Trust. A previous version of this story also misspelled her youngest child’s name, based on initial information from authorities. The correct spelling is Devin.

At a few minutes before 11 a.m., the day was well underway for a young mother, her infant son and his toddler brother when a twin-engine jet, on its final approach to a regional airport in Gaithersburg, crashed into their home.

The plane caromed into two homes before its wing tumbled into 19733 Drop Forge Lane, erupting into a fireball. The pilot, two passengers, the mother and her children were killed.

As people rushed from their houses, Marie Gemmell, 36, had been in a desperate fight to save 3-year-old Cole and 6-week-old Devon. She rushed them into a windowless bathroom on the second floor.

“It appears the mother was trying to protect her children. It appears she was covering them to try to protect them,” said Montgomery fire spokesman Pete Piringer.

“She tried to save these kids,” Police Chief J. Thomas Manger said of the mother of three, who friends said was on maternity leave from her job at a bank.

“She had nowhere to go,” he added. “She couldn’t get out of the bathroom. One kid was between her legs, and the other was in her arms.”

(Air traffic control audio: ‘There’s nothing left’ of this house)

Within seconds of the crash, another pilot looked down in horror. “There’s nothing left of that house down there,” he said.

The black Honda SUV parked snugly against the garage door indicated that there probably were people inside the house, and neighbors rushed around the outside, looking for an opening in the flames through which they might dart in.

The plane lay on the front lawn in pieces, as though “it just fell apart,” Jocelyn Brown, 21, said.

“It’s like somebody took the screws out,” she said.

With a flight data recorder in hand, National Transportation Safety Board investigators should quickly determine what caused the crash, although their methodical process may not reveal the telling details for several months.

What would become tragedy on Drop Forge Lane began to unfold hours earlier, when a small jet plane — an Embraer EMB-500/Phenom 100 — set out from North Carolina to carry three members of the medical research firm Health Decisions to an important meeting with Food and Drug Administration officials in Montgomery County.

Before Michael Rosenberg set out to fly to Maryland, he called his company’s technology department to sort out a Web site issue.

(Video: Police say death toll climbs to six)

“He sounded great,” said an IT worker. “It was just standard tech support stuff. A minor issue, really.”

He was a veteran pilot — he was certified to fly commercial planes and to teach novice pilots — but it was the second time that Rosenberg had crashed while landing at the Montgomery County Airpark. In March 2010, he suffered minor injuries when he crashed a Socata TBM 700, a turbo-propeller plane. He was coming from North Carolina on that trip, too.

On Monday, his voice sounded matter-of-fact as he talked with air traffic controllers — using his call sign, 100 Echo Quebec — while approaching Runway 14.

“Montgomery traffic, 100 Echo Quebec is now 7 miles” from landing.

“Montgomery traffic, 100 Echo Quebec is now 6,000 [feet], straight” toward the runway.

“Montgomery traffic, 100 Echo Quebec is 3 [miles] out, straight in [toward] 1-4.”

Then came a call from another pilot who saw what happened:

“We got a Phenom [jet] crash at the end of the runway.”

The airport is an uncontrolled runway, which means that, as opposed to larger airports, there is no air traffic control tower directing final approaches. The county-owned airport opened in 1959 to relieve aviation traffic into what is now Reagan National Airport. Since the emergence of Baltimore-Washington International Marshall Airport and Dulles International Airport, the facility in Gaithersburg has transformed into one used by small planes and business travelers.

The airport has about 100,000 annual departures and arrivals and is the fourth-busiest general aviation airport in Maryland.

There have been two accidents at the Gaithersburg airport this year. On Sept. 13, a U.S. Coast Guard Auxiliary Cessna nosed over after landing. The pilot and two passengers were not injured. Three weeks earlier, the pilot of a Piper plane was seriously injured when he made a forced landing after his engine failed. There have been 12 crashes at the airpark since 1996, none of them fatal.

On Monday morning, Tracy Everett said he was driving his work van when he looked up and saw that the plane was “unsteady” and in trouble. “It was wobbly,” he said. “It was 100 to 200 above the trees.” He said the plane then did a rolling dive to the left, and then “I saw smoke.”

He said he drove to the scene and “saw and heard a secondary explosion. It was so powerful you could feel it under your feet.”

Dianne Gayle, who lives on the street, said she heard a plane overhead as she was working in her living room. That’s not unusual, given how close she and her neighbors live to the airport.

Then she heard a boom, and her house seemed to shake. She jumped up, looked out her window and saw a home down the street engulfed in flames. Gayle called 911. “The house is on fire! The house is on fire!” she remembered telling the operator. “A plane crashed into the house! A plane crashed into the house!”

Gayle walked outside. “It was a total inferno,” she remembered.

Gayle spotted cars in the driveway of the burning house and desperately hoped no one was home. “Dear God, don’t let them be inside,” she said.

In an interview later, Gayle’s husband, O’Neil, said he saw the Gemmells out on walks, and saw them doting on their children. “They are a loving, friendly family,” O’Neil Gayle said.

Byron Valencia, 31, who lives about a half-mile from the crash site, said he was in his kitchen preparing a bottle for his 2-month-old son. “I heard the plane come over the house,” he said. “This one sounded like a jet, and then I heard a thump. It was pretty loud. I didn’t see anything, but then I heard the sirens.”

Miriam Arevalo, who lives with her husband and two children a block from the crash site, was home waiting for a ride to work when she heard a big boom.

“I heard a big explosion. It’s like nothing I’ve ever heard before,” she said. “I immediately thought, ‘Oh my God, it is a plane.’ ”

At the FDA offices, James Higgins, executive vice president of Health Decisions, waited anxiously for Rosenberg and two other colleagues to arrive. Instead, he got word of the crash.

“He’s in shock,” said Barbara Higgins, his wife, “and he’s also trying to figure out what they’re going to do on their end.”

The Gemmells have one other child, a girl, who was at school at the time of the crash. Neighbors said Ken and Marie Gemmell had lived in the house for about seven years and were known to dote on their children, who often were seen playing in the front yard. Ken Gemmell had left the house a few hours before the crash. After rushing back, he was led to the home, knowing that his wife and children had not emerged. As the house still was in flames, he stood in front of it, staring blankly for 10 minutes before being led away.

Montgomery Fire Chief Steve Lohr spoke quietly to him.

“We’re doing everything we can to determine the location of your family,” Lohr told him.

Late Monday, Ken Gemmell had changed his Facebook profile photo to one of his wife, Devon and Cole. Another photo showed him and his wife at a festive occasion. Several friends had posted notes of condolence.

A former colleague of Marie Gemmell, who worked at Davis Construction for a dozen years before taking a job several months ago at a bank, mourned her loss.

“She’s really going to be missed,” Brian Polesnak said of his friend, whose Facebook page said she was a native of New Jersey. “She always loved her family, loved her kids. She always had a smile or a joke.”

He is marrying his girlfriend Saturday and said he had wanted Gemmell to be there.

Alice Crites, Dana Hedgpeth, Luz Lazo, Miles Parks, Michael S. Rosenwald and Julie Zauzmer contributed to this report.