OMAHA, Nebraska (CNN) -- Investigators probing the deadly mall shooting in Omaha have seized computers and are analyzing information on Web sites in the search for clues in the case, police said Thursday.
The 19-year-old gunman sent at least one text message to his former girlfriend, with whom he broke up about two weeks ago, Omaha Police Chief Thomas Warren said.
Robert Hawkins also left a voice mail for his mother before killing eight people at a department store Wednesday.
He chose his victims randomly, police believe, then took his own life.
Hawkins visited a friend near the Westroads Mall before the shootings and apparently went directly to the shopping center after that meeting, Warren said.
Police don't yet know why Hawkins chose the mall or the Von Maur department store in particular, Warren said, adding only that the teen "may have frequented the Westroads Mall." See who Hawkins killed in department store »
"Typically there are hints that something like this may take place," Warren said. "Certainly you can't anticipate someone engaging in this type of shooter rampage, but if there is any justification, any explanation," police will find it, he said.
"Apparently he had been experiencing some mental health problems, ideations of suicide."
"He basically said how sorry he was for everything," she told CNN's Anderson Cooper on Wednesday. "He didn't want to be a burden to people and that he was a piece of s--- all of his life and that now he'd be famous."
Warren said the rifle used was an AK-47. Police haven't finished tracing the gun, but believe Hawkins stole it from his stepfather's home.
Security officers flagged Hawkins as suspicious when he entered the mall. He left soon after he entered, then returned less than six minutes later with something apparently hidden in a balled-up sweatshirt.
He went up an elevator to the mall's third floor, then immediately began firing, ultimately turning the firearm on himself, Warren said.
"It doesn't appear as though there was an opportunity for intervention," Warren said.
Hawkins fired more than 30 rounds, the police chief said.
The shootings sent panicked holiday shoppers fleeing for cover.
"It was just so loud, and then it was silence," said witness Jennifer Kramer, who hid inside a circular clothing rack. "I was scared to death he'd be walking around looking for someone else." Watch how Kramer and her mother hid »
A friend of Hawkins' said he hadn't thought Hawkins was capable of such violence.
"He was the one guy, you know, if people would be getting in a fight he'd be trying to break it up," said Shawn Saunders, who had known Hawkins for about 2½ years. "If there were arguments amongst our friends or groups, he was kind of like the calm, cool and collected one." Watch how Saunders learned Hawkins was the shooter »
Nebraska Gov. Dave Heineman said Hawkins had been a ward of the state for nearly four years, but he did not specify in what capacity. The state's custody ended in August of last year, Heineman said.
U.S. Army recruiters turned Hawkins down last summer when he tried to enlist, a source familiar with the situation said Thursday.
The source didn't want to be named because it is against the military's rules to discuss potential recruits. The reason for his rejection was unclear.
Heineman ordered that flags throughout the state be lowered to half-staff through Sunday.
The dead include six store employees and two customers, ranging in age from 24 to 66.
One store employee was in critical but stable condition Thursday and another had been upgraded from critical to serious, according to hospital officials.
Another person was still being treated Thursday afternoon, Warren said, but he did not specify whether the individual was a customer or employee. Two other customers had been treated and released, he said.
Westroads Mall remained closed Thursday, but the facility, including the Von Maur store, could reopen Friday. E-mail to a friend
CNN's Kevin Bohn contributed to this report.