(CNN) -- The chief of Houston's police department has asked the FBI to help investigate why an officer shot and killed a disabled man Saturday.
Over the weekend, police said that the wheelchair-bound man was acting aggressively and had tried to stab an officer with a pen.
"The Houston Police Department places the highest value on human life and events like these are tragic and unfortunate for everyone involved," Chief Charles A. McClelland said in a press release Monday afternoon. "All Houston Police Officers receive mandatory crisis intervention training specifically dealing with persons experiencing mental crisis. As we do in all instances of this nature, the Houston Police Department's Homicide and Internal Affairs Divisions, and the Harris County District Attorneys Office, Civil Rights Division, are investigating this incident."
McClelland vowed to be "open and transparent in all aspects of our response to this tragic event."
"It is my desire to have everyone reserve judgment until all the facts and evidence in this investigation have been gathered," his statement read.
On Saturday officers went to a group home for the mentally ill where Brian Claunch lived. A reported schizophrenic, Claunch was acting aggressively because his caretaker refused to give him a cigarette and a soda, according to police and the facility owner.
Police said Claunch was clutching an object as he advanced on one of the police officers.
"The officers made verbal commands for the suspect to drop whatever he had in his hand, to stay still and to speak with the officers, but the suspect continued to make threats," Jodi Silva, a police spokeswoman, told CNN affiliate KTRK in Houston.
Claunch trapped one officer with his wheelchair in the corner of a room "where he couldn't get out," said a Houston police department representative who declined to be identified.
As he advanced toward the officers, Claunch was "refusing to show his hands," the representative said.
The object turned out to be a pen, which Claunch attempted to stab the cornered officer with, according to police accounts reported in the media, including by KTRK.
The other officer, Matt Marin, was "in fear of the safety of his partner and the safety of himself," Silva said.
Marin shot Claunch in the head, police said.
On Sunday, the unidentified police representative offered more details, saying that, unlike his partner, Marin was not cornered at the time he opened fire.
Claunch died at the scene, investigators. said
Marin has been placed on administrative leave per department policy, police said.
The incident was the second shooting involving Marin.
In October 2009, he shot and killed a knife-wielding man who stabbed his girlfriend and a neighbor, according to published reports at the time. Marin joined the Houston Police Department in 2007.
Claunch had been at the Healing Hands group home for 18 months, owner John Garcia told KTRK. He lost an arm and a leg in a train accident.
Houston police have not released the identity of the man, though Garcia identified him as Claunch.
Garcia told the Houston Chronicle that Claunch liked to "doodle," which may explain why he was holding a pen.
Garcia reportedly said he had given Claunch a black felt pen for drawing. Garcia told the Chronicle he did not know if that was the pen Claunch was clutching when he was shot.
Garcia said Claunch suffered from schizophrenia and a bipolar disorder.
"He had a temper. He could fly off once in awhile," he told CNN affiliate KHOU.
Claunch was capable of flying into outrage and making people in the house feel threatened, even though he was confined to a wheelchair, Garcia said.
"Emotionally disturbed individuals, when threatened, are going to react, in most instances, excessively," Dr. Ed Reitman, a clinical psychologist, told KTRK.
"This was an incident that didn't have to take place if the individual -- a police officer -- had been trained in dealing with emotionally disturbed individuals."
CNN's Nick Valencia contributed to this report.