The Federal Bureau of Investigation (FBI) said Monday that it is investigating the police shooting of Brian Claunch, a wheelchair-bound double amputee who was reportedly armed only with a ballpoint pen, according to The Associated Press.

"In addition to the usual internal review, [Houston Police Chief Charles A. McClelland, Jr.] 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," Houston Mayor Annise Parker said Monday in prepared text. "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."

Officer Matthew Marin shot the 45-year-old Claunch in the head, killing him instantly. Police later said the incident, which took place around 1:30 a.m. on Saturday morning, happened because the officer wasn't sure what the man was holding and feared for his partner's life. Both officers were called to Houston's Healing Hands Assisted Home Care on a domestic disturbance call after a caretaker said Claunch flew into a rage over being denied cigarettes and soda.

The man had only one arm and one leg, having lost the others in a train accident. The group home's owner also told ABC News affiliate KRTK in Houston that Claunch suffered from bipolar disorder and schizophrenia.

Houston Police Department spokesperson Jodi Silva told reporters that the man threatened officers and "waved a shiny object in his hand," then "backed one of the officers into a corner" and attempted to stab him. "Officer Marin, fearing for his partner's life, and his own safety, discharged his duty weapon one time, striking the suspect," she added. "The object was discovered to be a shiny, ball point pen."

Claunch's death marks the second time Officer Martin has killed a suspect in the line of duty. The five-year veteran shot and killed a knife-wielding man who stabbed two others in 2009. He's since been placed on administrative leave pending an investigation.

Community activists with the Texas Civil Rights Project-Houston and the Greater Houston Coalition for Justice said they planned to protest the killing on Tuesday at city hall. Both groups have asked for a public review of the facts in Claunch's death.

This video is from ABC 13 in Houston, published Tuesday, Sept. 25, 2012.