HOUSTON (AP) — A caretaker at a group home for the mentally ill called police in the middle of the night because a one-armed, one-legged man in a wheelchair was angry and wouldn't calm down. What happened next is the subject of an investigation that now involves the FBI. One of the two Houston police officers who entered the home fatally shot the double-amputee in the head, saying he was cornered by the wheelchair and thought his partner was being threatened by what turned out to be a ballpoint pen. Houston's police chief responded Monday to escalating criticism about the weekend shooting by cautioning against a rush to judgment. Community and civil rights groups say the incident is another example of problems the police department has with using excessive force. "It is my desire to have everyone reserve judgment until all the facts and evidence in this investigation have been gathered," Houston Police Chief Charles McClelland said. Officer Matthew Marin shot 45-year-old Brian Claunch early Saturday after responding to a call that the man, who reportedly lost two limbs in a train accident, was causing a disturbance. Police say Claunch cornered and threatened to kill Marin, who reportedly told investigators he didn't know the object in Claunch's hand was a pen. John Garcia, who owns the group home, told reporters over the weekend that Claunch liked to draw. McClelland said Monday he would enlist the FBI's help in the investigation and reassured the public his officers are trained to deal with people with mental problems. Police spokeswoman Jodi Silva said she didn't know if the department requesting FBI assistance in officer-involved shootings was rare but said "it's the step we're taking at this point." But the Greater Houston Coalition for Justice, a group that includes 16 local and national civil rights organizations, suggested Claunch's death was part of a bigger problem at the police department. "The deeper problem is a failure to discipline for excessive force, especially in the area of shootings," said Randall Kallinen, a member of the group and a local civil rights attorney. "They are concerned only about liability." Kallinen said he would like the shooting to spark a change in the police department regarding discipline and training of officers. Marin, a five-year veteran of the department, has been placed on three-day administrative leave. That is standard department procedure for all officer-involved shootings, and Silva said no unusual measures were being taken that would prevent Marin from returning to duty this week. It's the second time Marin has killed a suspect while on duty. In 2009, investigators said Marin came upon a man stabbing his neighbor to death at an apartment complex and fired when the suspect refused to drop the knife, according to the Houston Chronicle. In 2008, Marin was one of three Houston police officers who were accused of assaulting the father of Green Bay Packers player Donald Driver. They were later cleared and filed a defamation lawsuit that was later dropped. The Healing Hands group facility, located in a somewhat rundown brick home, was quiet Monday. Garcia could not be found at the facility on Monday and a woman at a separate address under his name said he was not there and asked a reporter to leave the property. Estella Olguin, a spokeswoman for the Harris County Guardianship Program, said Claunch had been a ward of the country since 2003 and had lived at the home since March. Garcia said Claunch had lost his right leg to just above the knee and his entire right arm when he was hit by a train, according to the Houston Chronicle. He said Claunch was schizophrenic. Marin and a partner arrived at the Healing Hands around 2:30 a.m. responding to a disturbance call made a caretaker at the home. Police have not elaborated on the nature of the disturbance and authorities Monday had not yet made the 911 call public. Once inside the home, according to a department statement Monday, the two policemen found Claunch agitated and threatening to kill the officers and other residents. While yelling at the officers, Claunch "waved a shiny object in his hand in their direction," according to the statement. Police say Claunch refused an officer's direction to drop what turned out to be a ballpoint pen. "As the suspect backed one of the officers into a corner, he attempted to stab the officer with the object," the statement said. "Officer Marin, fearing for his partner's life, and his own safety, discharged his duty weapon one time, striking the suspect." Relatives of Claunch could not be found Monday. Public records show Claunch had brushes with the law multiple times over the last 20 years, including a 2006 arrest for trespassing at a shelter. His record also includes drug convictions.