A man accused of gunning down two Wal-Mart employees in a store parking lot -- reloading and continuing to fire into the men after they fell -- did not appear to know either victim and had no known vendetta against the company, authorities said.

Court papers released Wednesday indicated the suspect, Ed Liu, 53, was mentally disturbed but gave no further details.

The two employees had been collecting shopping carts in the crowded parking lot of their suburban Phoenix store Tuesday afternoon when a man drove up and started shooting, according to authorities.

Court documents released after Liu's initial court appearance Wednesday alleged that after shooting the two, Liu reloaded his .40-caliber handgun and then shot them several more times as they lay on the ground about 75 yards from the store entrance.

Both victims, Anthony Spangler, 18, and Patrick Graham, 36, were wearing their blue Wal-Mart vests, and both died from their wounds.

Two witnesses to the shooting gave police the license number of the shooter's car, and Liu was arrested a few hours later in a nearby retirement community where he lived. He was booked late Tuesday on two counts of first-degree murder.

Liu did not immediately respond Wednesday to a request through jail officials for an interview. A search of criminal record databases turned up no criminal history for him in Arizona.

Neighbors said they rarely saw Liu, who lived in the neighborhood for about seven years.

"He was just a quiet guy and one of those people you would least suspect," said neighbor Judy Devlin.

A Glendale police spokesman, Officer Mike Pena, declined to comment on why investigators believe Liu is mentally disturbed, saying he did not want to jeopardize the case. He said authorities do not believe Liu knew the victims or held any grudge against Wal-Mart.

Graham and Spangler had worked at the store only since the first week of August, said Delia Garcia, a Wal-Mart spokeswoman. "Everyone's been deeply affected by the loss," she said.

Graham was married and had two boys, ages 10 and 11, Garcia said. Spangler recently graduated from high school and lived with his grandparents, said Natasha Fox, a friend.

Spangler "wouldn't hurt a fly," Fox said. "He always looked out for us."