JACKSONVILLE, Fla. -- Despite taking several shocks from two police officers' Taser guns, a mentally ill man got up and kept coming at them with two steak knives, police said one day after the man was shot and killed.

The incident began just before 11 a.m. Tuesday when a caseworker making a daily check on 30-year-old Sierra White at his Arlington apartment met resistance and called for police assistance.

With White's behavior increasingly erratic and threatening, he went back inside his apartment and officers said they thought they heard the sound of a gun being "racked," or loaded. The officers took cover but continued to try to talk to White.

The incident escalated when White withdrew two knives from his pockets -- one in each hand -- and began threatening the officers, police said.

Two officers deployed their Taser guns, but repeated shocks appeared to have no effect. Police said that when White got up and threatened a police sergeant, he was shot four times.

"He was up, making a move in a threatening manner toward the officer when the officer used the lethal force," Graham said.

White died at the scene.

"A knife is considered a lethal weapon. A Taser is non-lethal," Graham said. "If the defendant has a knife ... they would have been well within state law to go immediately to their firearms. That was just a conscious decision by our officers to try to do everything possible to keep from having to go to lethal force."

A realistic-looking BB-gun was found in the victim's pocket, but police said they found no "true firearms" in his apartment.

At a news conference on Wednesday, Graham said White had a criminal history, including burglary and an assault with a deadly weapon.

"He's been known to carry a knife," Graham said.

An examination of the Taser gun found that one officer pulled the trigger four times and another officer fired five shocks from his Taser gun. Graham said it's possible the probes were not making contact or became dislodged.

The Jacksonville Sheriff's Office and the state attorney's office were conducting separate investigations whether appropriate force was used in the incident.

This is the eighth police-involved shooting of 2008 and the fifth person to have died in the incidents. One police officer was shot in the altercations. That officer is recovering from his injuries.

"We hope that we can get through our career and not have to take somebody's life, but if we have to do it, it's part of the job," Graham said.