Fairfax County law enforcement officials investigate near a home where a family of four was found dead Tuesday. (Dayna Smith/FOR THE WASHINGTON POST)

A 57-year-old Herndon man fatally shot his wife and two teenage sons before killing himself in their home, and detectives were still working to determine what may have prompted the violence, Fairfax County police said Wednesday.

Police identified the man as Albert Peterson, his wife as Kathleen Peterson, 52, and their children as Matthew, 16, and Christopher, 13. They were found dead Tuesday afternoon.

Kathleen Peterson worked at a Herndon technology company that specializes in defense, law enforcement and intelligence work. Albert Peterson worked for a defense contractor, neighbors said. Matthew was a sophomore at Westfield High School, and Christopher was in the eighth grade at Rachel Carson Middle School.

Relatives and friends mourned the family and struggled to come to terms with what had happened. Neighbors, who said they hadn’t noticed anything out of the ordinary in recent weeks, said that the four often sat on their porch and grilled on a barbecue and that the sons played soccer in the front yard.

At Westfield High on Wednesday morning, dozens of students linked hands in an impromptu memorial. Some at the school wore black shirts, and others posted remembrances on Twitter. One read: “matt rode my bus and I never paid much attention to him. really wish I did now.”

Christopher Peterson, 13, one of the teenage sons killed in Herndon murder-suicide. (Courtesy of Fairfax County schools)

Matt Peters, 16, who knew Matthew Peterson from a church youth group, described him as a “funny kid” who always had a joke and a good attitude.

On Wednesday evening, hundreds of people came to a candlelight vigil at Floris United Methodist Church, where the family had attended services. Mourners sang “Amazing Grace” and wept.

“Our community feels a real sense of loss at this time,” the Rev. Tom Berlin said earlier. “This is a family that people cared about.”

Natallya Blackshear, 12, who attended school with Christopher, said she planned to play her next soccer game in his honor. She said she and other classmates wore black Wednesday and planned to wear white on Thursday as a sign of peace.

Randall Alter, a neighbor who said he has known the Petersons for about 28 years and attended their wedding and their sons’ baptisms, said Albert Peterson stopped by his house Saturday night.

His friend, Alter said, seemed “morose.”

“He was worried about everything,” Alter said. “He felt the economy was going in the wrong direction. He was worried about politics. He couldn’t shake things.”

Still, he said, Peterson often seemed negative and Alter never imagined what was to come. He said Peterson had not mentioned any family disputes or financial problems.

Alter said he walked his dog about 3 a.m. Monday and noticed that the lights were on in the Peterson house and garage. About an hour later, he said, he and his wife heard a muffled bang.

Officer Don Gotthardt, a county police spokesman, said detectives were still unraveling a motive. He said the Petersons were seen alive as late as 5 p.m. Sunday.

On Tuesday morning, police said, co-workers of one parent called police and said the parent didn’t show up for work Monday or Tuesday. Officers went to the home and found the family dead.

Family members reached Wednesday declined to comment.

Blackbird Technologies, the technology company where Kathleen Peterson worked, released a statement from chief executive Peggy Styer.

Kathleen Peterson “will be remembered as a high energy, devoted parent,” Styer said. “She often told stories about her sons, their sports and community activities. She is already missed.”

Jennifer Jenkins and T. Rees Shapiro contributed to this report.