HOUSTON -

Activists voiced their outrage on Tuesday afternoon at City Hall after a mentally-ill man, who was also a double-amputee, was shot and killed by a Houston police officer.

The officer involved, five-year veteran Mathew Marin, said he felt his life was in danger when he opened fire on Brian Claunch, 45.

Marin shot and killed Claunch inside his group home on Polk and Sidney in southeast Houston Saturday at 1:30 a.m. Police said they were called out to the home when Claunch became agitated after his caregiver would not give him a soda and a cigarette.

Activists are questioning the officer's actions. They believe it was excessive force.

"When officers arrived and went inside to speak with him, he was situated in a wheelchair and holding an unknown object," said Houston Police Department spokesperson Jodie Silva.

The unknown object turned out to be a pen. Investigators said Claunch repeatedly threatened an officer in his room. Police said when Claunch refused police orders, Marin shot him in the head.

"How in the hell can a man who is a double-amputee with a writing pen be such a threat?" community activist Quanell X said.

Houston police records indicate that Marin also fatally shot a suspect in 2009. Investigators at the time said Marin came upon a man stabbing his neighbor to death at an apartment complex and opened fired when the suspect refused to drop the knife. According to state records, Marin has been trained in crisis intervention and mental health situations.

The Police Chief issued a statement Monday.

"The Houston Police Department places the highest value on human life and events like these are tragic and unfortunate for everyone involved."

McClelland said HPD's Homicide and Internal Affairs Division, the Harris County District Attorney's Office's Civil Rights Division and the FBI are investigating the shooting. 

Houston Mayor Annise Parker also released a statement.

"I want to express my condolences to Mr. Claunch's family and friends. As for any comment on the circumstances, there is a process in place to determine if the officer acted appropriately. In addition to the usual internal review, Chief McClelland has taken the additional step of asking the local office of the Federal Bureau of Investigation (FBI) within the U.S. Department of Justice to monitor our investigation of this incident. Until the facts are in place, it is premature to draw any conclusions. I have utmost respect for the very difficult job of our Houston Police Department.  However, if there were mistakes made, I know the police chief will take appropriate action," Parker said.

Records show Claunch was diagnosed with schizophrenia and bipolar disorder. 

A prayer vigil was scheduled for 6:30 p.m. in front of the group home.