Thomas Mortimer IV, 43, was captured Thursday by police in northwestern Massachusetts hours after he was charged with four counts of first-degree murder.
There were signs of attempted suicide at the home, and two identical letters, believed to have been written by Mortimer, were left behind, District Attorney Gerry Leone said.
"I did these horrible things. What I've done was extremely selfish and cowardly. I murdered my family," the letters read.
Leone said the slayings followed a fight and "ongoing marital discord."
Authorities were summoned to the home by Laura Mortimer's sister after she could not reach Laura. Officers found Finn and his mother lying in a pool of blood in the first-floor hallway.
The grandmother's body was nearby covered with an oriental rug, and a trail of blood led to an upstairs crib, where they found Charlotte dead.
All appeared to have been killed by blunt trauma with sharp objects, Leone said.
The discovery of the bodies prompted a search for Mortimer, with police issuing alerts about his vehicle on electronic signs around the state.
A father and son spotted his SUV late Thursday morning in Montague and called police.
He was captured after trying to flee in Bernardston, about 100 miles from Boston.
Leone said the exact time of the slayings was still being determined but appeared to be sometime between late Monday and early Tuesday.
Mortimer called in sick to work on Tuesday, and called his son's school to say he would not be in.
Leone said Laura Mortimer's sister, Debra Stone, tried to call her Tuesday, but instead Thomas Mortimer answered her cell phone - which was highly unusual.
Thomas Mortimer told Stone, "It's going to be a while before she can get back to you," Leone said.
Mortimer had recently landed at job at M&R Consultants Corp., a Burlington technology consulting firm, after several months of unemployment, said Anil Shah, the company's president.
Mortimer was a hard worker who had been making progress at his job since getting hired about a month and a half ago, Shah said.
"He was very professional, very nice guy ... always very positive," Shah said. "Somehow my heart doesn't believe he could be involved in anything that he's been charged with."
Mortimer had left a message for his supervisor around 7:30 a.m. Tuesday to say he wasn't feeling well and wouldn't be at work, Shah said.
About two hours later, Mortimer told a co-worker he had been up sick all night and would be back at work on Wednesday, Shah said.
Laura Mortimer was a senior economist with the Los Angeles-based commercial real estate firm CB Richard Ellis. The company called her death a tragic loss.
"Laura was a valued and well respected colleague and, more important, a good friend," the company said in a statement. "Our thoughts and prayers are with Laura's family and loved ones at this terrible time."