Late last week, the Gemmells bundled up and went to a Christmas tree lighting in downtown Rockville: Marie, Ken and their kids, Arabelle, 7, Cole, 3, and even little Devin, not yet two months old.

Arabelle hit the outdoor ice rink. Cole got to meet Santa. Marie stood to the side, holding Devin.

“That was them,” said Pallavi Rana, a close friend. “They were always together.”

The family of five became two Monday morning in the most random of tragedies. A small jet trying to land at an airport near their Montgomery County community crashed in a neighbor’s yard, sending a fuel-laden wing catapulting through the side of the Gemmells’ house.

At the time, Ken was at work; Arabelle was at school.

Marie Gemmell, 36, with her son Cole. (Courtesy of Ken Gemmell)

Marie, Cole and Devin were home.

Investigators believe that the three were upstairs. From below, there was a sudden, furious eruption of flames, fed by jet fuel mixed with oxygen rushing into the house through large holes the wing had created.

(Audio: 911 calls after crash: ‘It went straight down’)

“The fire was immediate and intense,” said Capt. Darren Francke, commander of the Montgomery Police Department’s major crimes unit.

Marie made her way to a bathroom with Devin and Cole, detectives believe, and held one of the boys in her arms and the other between her legs. It was probably just a matter of minutes, authorities said, before all three died of smoke inhalation.

“She took that bathroom as an area of refuge,” said Capt. Michael Redding, a fire investigations supervisor in the county. “To be trapped like that, it’s terrible.”

As friends and relatives learned what Marie did — covering her children, trying to save them — they expressed no surprise.

“She loved those kids more than anything on the face of the Earth,” said John Sadlik, a friend.

Until Monday, the Gemmells’ life seemed defined by smiles and cheers. They hosted parties. They took the children to D.C. United soccer games. They liked to eat together at the Dogfish Head Alehouse in Gaithersburg. Marie wouldn’t just say, “Hi,” to waitresses, said Haleigh Powell, who works there. She would say, “Sit down and have a beer with us,” according to Powell.

Ken and Marie, natives of New Jersey, were married Sept. 25, 2004, according to their Facebook pages.

Marie would write about it a decade later, while pregnant with Devin: “10 years ago today. It was bright, clear sky and warm and I was likely having a mimosa right now. Today 2.5 kids, a dog, turtle and a house later its rainy and cold and I’m on my way to work with no mimosa. No matter the weather or what we are doing its still a great a day. Happy 10-year Anniversary Ken Gemmell.”

Ken Gemmell’s Facebook page also tells the story of a family deeply entrenched in modern family life: soccer practices, football games, garden tomato harvests, pride in a daughter’s progress in reading.

“So in an effort to slow Arabelle down in her reading because she reads too fast and doesn’t always grasp all the concepts,” Ken wrote, “I have given her my college Calculus book. Strangely she took it without an argument, and now she wants to trace some of the shapes and graphs.”

Marie replied: “There is no doubt she is your kid. Her love and great skills at math make me wonder if she is mine.”

As Cole spent time at the Cuddles and Crayons day-care center, near their home in Gaithersburg, Marie became close with Rana, who ran the place.

And this month, Cole was on his way to being a successful student “at a big-boy school,” as he liked to say. His favorite color was blue, and he always wanted to sit in a blue chair and eat from blue plates. He was potty-trained last week, and he was proud.

“He told his mommy he wanted 10 pairs of underwear,” Rana said. “He was so smart. We always said he’s either going to be an engineer or an architect.”

With the arrival of Devin — and with Marie staying home with him — the Gemmells decided to reduce Cole’s day care to part-time — just Tuesdays and Thursdays. He was at home Monday for the first time after the switch, Rana said.

On Monday, as news of the crash spread, Rana hurried toward it, and she saw the Gemmell’s house in flames. “It was really confusing,” she recalled. “We didn’t know where anyone was. We were all lost. The basement light was on, so we were hoping they were in the basement. We were living in hope.”

Ken also rushed to the house and arrived while it was still in flames. Firefighters told him that they couldn’t account for his wife and sons.

Three people aboard the plane also died. All were from North Carolina: Michael Rosenberg, 66, David Hartman, 52, and Chijioke Ogbuka, 31.

(Video: Police say death toll climbs to six)

Rana was able to get Arabelle and bring her to the day-care center, where a relative picked her up.

“We tried to talk to her normally,” said Rana, who thought that the family should tell the little girl what happened.

Jill Lyons, who has known the Gemmell family for several years, said Marie enjoyed entertaining — for friends and for children in the neighborhood. When Cole turned 3, she threw an Elmo-themed birthday party. For Halloween, the Gemmell children dressed up as characters from the movie “Frozen.”

Brian Polesnak of Clarksburg said Marie Gemmell was always glad to help others, whether that meant organizing a happy hour to raise money for Hurricane Sandy victims or accompanying him to Tiffany’s when he was not sure how to pick out a first-anniversary gift for his girlfriend.

Just last week, Polesnak said, he talked to Marie about the Christmas trip to New York she was planning to take with her family.

Alice Crites, Dana Hedgpeth and Julie Zauzmer contributed to this report.

Anyone wishing to donate to funds set up for the Gemmell family can visitwww.gofundme.com/gemmellfamily.