MADISON, Nebraska (AP) -- A state trooper committed suicide Friday, apparently upset because he botched a background check involving a man who one week later allegedly killed five people in a bank robbery, authorities said.

Mark Zach, 35, shot himself with his service revolver just outside Norfolk, the site of Thursday's deadly heist.

Zach had stopped one of the four robbery suspects last week and ticketed him for carrying a concealed weapon, Gov. Mike Johanns said.

However, Zach transposed two digits when entering the gun's serial number into a police computer -- a mistake that kept him from learning the weapon had been stolen, Johanns said.

The gun was confiscated by authorities, but Zach apparently felt responsible for not getting the suspect behind bars on a more serious stolen weapons charge, said Col. Tom Nesbitt, Nebraska State Patrol superintendent.

"Trooper Zach could not accept that," Nesbitt said. "He took his responsibility very, very seriously."

Nesbitt did not elaborate as to how he knew how Zach was feeling. It wasn't known whether a more serious charge would have put the man behind bars or for how long.

Zach was a 12-year veteran of the patrol and was based in Norfolk. He and his wife had six children, ranging in age from 4 to 15, Nesbitt said. Zach was elected this year as an area chairman for the State Troopers Association of Nebraska.

Zach stopped Erick Fernando Vela, 21, for a traffic violation September 19 and arrested him on a concealed weapons charge. Vela was released after posting bond.

Authorities say Vela and three other men stormed into a U.S. Bank branch in Norfolk on Thursday and killed four employees and a customer before fleeing. All have been captured and charged with first-degree murder. (Full story)

"Zach was in no way responsible for what happened in Norfolk yesterday," Johanns said. "This tragedy only compounds yesterday's tragedy."

Norfolk Mayor Gordon Adams said he believed the stress of such a disturbing case could lead a law officer to consider suicide.

"That's dreadful," Adams said. "It is the kind of fallout you get sometimes with these tragic events."

Norfolk held a crisis intervention session for police, fire and other emergency personnel Friday. It wasn't known how many people attended.