Authorities did not release a possible motive for the murder-suicide, though police said that Belcher and his girlfriend had been arguing recently. The two of them have an infant child.
Belcher thanked general manager Scott Pioli and coach Romeo Crennel before shooting himself in the parking lot outside the team's practice facility, police spokesman Darin Snapp said. Police had locked it down by mid-morning and reporters were confined to Arrowhead Drive just outside the gates.
The news cast a pall over the organization as it prepared to play the Carolina Panthers on Sunday. There was no official word whether the game would be played as scheduled.
Belcher was a 25-year-old native of West Babylon, N.Y., on Long Island, who played college ball at Maine. He signed with the Chiefs an undrafted free agent but had stuck around for four years, moving into the starting lineup. He'd played in all 11 games this season.
"I struggle a little bit, obviously, because Jovan Belcher's profile elevates the subject," Kansas City Mayor Sly James said. "I hope people will look at the situation and try not to judge the person. There are a lot of people hurting. There's a young baby right now without parents."
Authorities reported receiving a call Saturday morning from a woman who said her daughter had been shot multiple times at a residence about five miles away from the Arrowhead complex.
"When we arrived, a lady informed us that her daughter had been shot multiple times by her boyfriend, by the daughter's boyfriend," Snapp said.
Snapp said a call was then received from the Chiefs' facility.
"The description matched the suspect description from that other address. We kind of knew what we were dealing with," Snapp said. The player was "holding a gun to his head" as he stood in front of the front doors of the practice facility.
"And there were Pioli and Crennel and another coach or employee was standing outside and appeared to be talking to him. It appeared they were talking to the suspect," Snapp said. "The suspect began to walk in the opposite direction of the coaches and the officers and that's when they heard the gunshot. It appears he took his own life."
The coaches told police they never felt in any danger, Snapp said.
"They said the player was actually thanking them for everything they'd done for him," he said. "They were just talking to him and he was thanking them and everything. That's when he walked away and shot himself."
Snapp described the girlfriend as in her early 20s and that she and the player had a child together. He said the woman's mother told police they had recently been arguing.
"We can confirm that there was an incident at Arrowhead earlier this morning," the Chiefs said in a statement. "We are cooperating with authorities in their investigation."
James said that he spoke to Pioli after the incident, and while he refused to discuss the GM's emotional state, the mayor said Pioli was "extremely concerned that fans of this team are not disappointed and not left in the cold."
"I think they think there's an obligation to the people of this city, the fans of the team and the fans of the other team to play the game," James said.
The season has been a massive disappointment for the Chiefs, who were expected to contend for the AFC West title. They're just 1-10 and mired in an eight-game losing streak marked by injuries, poor play and fan upheaval, with calls for Pioli and Crennel to be fired.
The Twitter account for a fan group known as "Save Our Chiefs" recently surpassed 80,000 followers, about 17,000 more than the announced crowd at a recent game.
Associated Press Writer Heather Hollingsworth contributed to this report.