MUNICH -- (CBS NEWS) The 18-year-old gunman who opened fire at a crowded Munichshopping mall and fast-food restaurant, killing nine people and wounding 16 others before killing himself, was obsessed with mass shootings, police said Saturday.

Investigators searched the unnamed German-Iranian man's home overnight and found a considerable amount of literature about mass killings, including a book titled "Rampage in Head: Why Students Kill," but no evidence that he was linked to extremist groups such as the Islamic State of Iraq and Syria, or ISIS. They believe he acted alone.

"Documents were found about mass shootings," Munich's police chief Hubertus Andrae told reporters. "The perpetrator was obviously obsessed with the issue."

Robert Heimberger, the head of Bavaria's criminal police, said it appeared the shooter had hacked a Facebook account and sent a message inviting people to come to the mall for a free giveaway.

The posting, sent from a young woman's account, urged people to come to the mall at 4 p.m., saying: "I'll give you something if you want, but not too expensive."

"It appears it was prepared by the suspect and then sent out," Heimberger said. The woman shortly after reported that her account had been hacked.

Initial investigations suggest the Munich-born suspect had suffered from psychological problems and received treatment, but details were still being confirmed, said Munich prosecutor Thomas Steinkraus-Koch.

The attack in the Bavarian capital sparked a massive security operation as authorities - already on edge after the recent attacks in Wuerzburg and Nice, France - received witness reports of multiple shooters carrying rifles shortly before 6 p.m. Eight hours later police declared a "cautious all clear," saying the suspect was among the 10 dead and that he had likely acted alone.

"We got one of the most critical injured patients," Dr. Peter Biberthaler, head of the trauma department at Rechts der Isar Hospital, told CBS News correspondent Seth Doane. "We tried to stop the bleeding, but the bleeding was not the problem. In this case, the traumatic brain injury was the problem because there were several bullets right into the brain."

His patient did not survive.

German Chancellor Angela Merkel was due to chair a meeting of her government's security Cabinet Saturday.

At an address on Dachauer Strasse that was searched by police early Saturday, a neighbor described the suspect as "very quiet."

"He only ever said 'hi'. His whole body language was of somebody who was very shy," said Stephan, a coffee shop owner who would only give his first name.

"He never came in to the cafe," he added. "He was just a neighbor and took out the trash but never talked."

Some 2,300 police from across Germany and neighboring Austria were scrambled in response to the attack, which happened less than a week after a 17-year-old Afghan asylum-seeker wounded five people in an ax-and-knife rampage that started on a regional train near the Bavarian city of Wuerzburg. ISIS claimed responsibility for the train attack, but authorities have said the teen - who was shot and killed by police - likely acted alone.

The suspect's body was found about 2 1/2 hours after the attack, which started at a McDonald's restaurant across the street from the mall. He was found with 9mm Glock pistol and at least 300 rounds of ammunition, police said.

A cellphone video posted online showed the person filming from a balcony engaging verbally with the suspect dressed in black standing on the rooftop of the mall parking structure. The shooter at one point yells, "I'm German," to which the filmer responds, "You are a jerk," and demands to know what he is up to. The shooter yells at him to stop filming, and shortly after begins shooting. Andrae said police believe the video is genuine.

Police have asked anyone with video and photos to upload them directly to their website to aid the investigation.

David Akhavan, a 37-year-old who from Tehran, Iran, who works at the Shandiz Persian restaurant, described his anguish as he learned of the shooting.

"I started to get texts from friends asking if I was safe," he said. "Then, my thoughts were: please, don't be a Muslim. Please don't be Middle Eastern. Please don't be Afghan. I don't accept any of this violence."

Witnesses had reported seeing three men with firearms near the Olympia Einkaufszentrum mall, but Andrae said two other people who fled the area were investigated but had "nothing to do with the incident."

Local residents described the scene as the shooting unfolded.

"I was standing on the balcony smoking a cigarette. Suddenly I heard shots," said Ferdinand Bozorgzad, who lives in a high-rise building next to Olympic Shopping Center. "First I thought someone had thrown some firecrackers. I looked down at the McDonald's and saw someone shooting into the crowd. Then I saw two people lying there."

Franco Augustini, another local resident, said his daughter hid in the shopping center during the attack.

"Next to our flat was a woman who was full of blood," Augustini said. "My wife had a bottle of water. Then we helped to wash her. It was horrible and made me speechless."

Andrae, the police chief, said seven of the victims were teenagers; a 20-year-old man and a 45-year-old woman were also killed. All were residents of Munich, he said. Twenty-seven people were hospitalized, including four with gunshot wounds, said Andrae.

He said the city was safe to visit and that the attack wasn't linked in any way to the recent influx of asylum-seekers that has stirred a debate about immigration in Germany,

Munich's mayor, Dieter Reiter, declared a day of mourning for the victims of "this terrible act."

"These are difficult hours for Munich," he said, adding that the city's citizens had shown great solidarity toward each other. "Our city stands united," he said.

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