There was no immediate indication from police or others what prompted the 25-year-old Belcher to shoot Kasandra Perkins, 22, with whom he had a three-month-old child, in the house they shared in Kansas City about two miles from the Chiefs' home field at Arrowhead Stadium.
Police spokesman Darin Snapp said Perkins's mother witnessed the killing and called police. Perkins had suffered multiple gunshot wounds and was pronounced dead at a hospital.
The mother told investigators that Perkins and Belcher had argued just before the shooting but that Belcher had never before been physically abusive with her daughter, Snapp added.
Belcher then drove his car to the team's training facility near the stadium, where he encountered head coach Romeo Crennel and general manager Scott Pioli, then shot himself in the head just as police arrived.
"As officers pulled up and were getting ready to get out of their car, they heard a gunshot," Snapp said. "The individual, it appears, took his own life."
Snapp said Belcher had made no threats to Crennel, Pioli or other employees when he arrived. "He was just talking to them and thanking them for everything they had done for him," the police spokesman said.
The suicide of Belcher followed a recent string of former National Football League players who have taken their own lives, including Junior Seau in May, Ray Easterling in April and Dave Duerson last year.
Those deaths heightened growing concerns about the risk of brain injury from repeated concussions suffered by veteran NFL players in a game that some critics say has grown too aggressive and brutal.
At a news conference later in the day, Kansas City Mayor Sly James decried the violence as "part of the tragedy of urban living in this country."
"Handguns all over the place, people blowing themselves away, and others. At some point, we have to get a handle on this kind of stuff. We are not doing a good job of it," he said.
He also expressed bewilderment at what drove Belcher to such violent behavior.
"A young man in a high-profile position, for whatever reason, felt the end of the world had come, and he had to act in the way he did," James said. "What kind of burden was he under to do that?"
Although news of the murder-suicide stunned the tight-knit NFL community, the Chiefs announced later their game against the Carolina Panthers at Arrowhead would be played on Sunday as scheduled.
The Chiefs have won just one of 11 games this season, the worst record in the NFL.
In a message on the social media network Twitter, Chiefs' tight end Tony Moeaki wrote: "Devastated. One of everyone's favorite team mates including one of mine. Great great great team mate. We will miss him forever."
Belcher was signed by the team in 2009 after he was overlooked in the NFL draft and established himself as a regular starter in his second season. Earlier this year, he signed a one-year deal worth just under $2 million. This season, Belcher started 10 of 11 games, making 38 tackles.
"Our thoughts and prayers are with the Chiefs and the families and friends of those who lost their lives in this terrible tragedy," NFL spokesman Greg Aiello said, adding that professional counsellors were being made available to team members and their families.
The Chiefs' chairman, Clark Hunt, issued a statement offering sympathy and condolences to "the families and friends affected by this unthinkable tragedy."
"We will continue to fully cooperate with the authorities and work to ensure that the appropriate counseling resources are available to all members of the organization," he wrote.
Condolences from others throughout the NFL sprang up on Twitter, where NFL Players Association assistant executive George Atallah wrote, "There is nothing profound or comforting to say that can help us understand or explain a situation like this."
Defensive end Justin Tuck, a two-time Super Bowl winner with the New York Giants, tweeted: "Man prayers go out to the KC Chiefs community and families after this morning’s tragic incident."