Munich police say they have arrested the 16-year-old friend of the 18-year-old shooter who killed nine people at a shopping mall on Friday.

The 16-year-old, a friend of Munich shooter David Ali Sonboly, is an Afghan who is being investigated for not reporting Sonboly’s plans to kill others to the authorities, according to the BBC News. Munich police also believe the young friend of the shooter could be an accomplice.

A statement on Munich police’s Facebook page says, “There is a suspicion that the 16-year-old is a possible tacit accomplice to the attack.”

The Facebook statement from the Munich authorities says the young friend of the shooter reported to police immediately after the deadly incident on Friday. The 16-year-old was initially interviewed as someone who simply had a connection to the shooter, but soon authorities uncovered discrepancies in the teenager’s statements.

The teenager connected to the Munich shooter is now being investigated on suspicion of “failing to report a planned crime.”

It is unclear whether the authorities believe that the Afghan teen could be involved in a Facebook post that invited people to gather at a cinema complex near the main railway station in Munich.

It is still unclear whether the Munich shooter tried to lure people to this McDonald's in order to kill him, and whether he had an accomplice. (AP Photo/Jens Meyer)

CNN reports that German Interior Minister Thomas de Maiziere said the shooter may have hacked a woman’s Facebook account before the attack to post a fake free food offer in order to lure people to the McDonald’s where the shooting occurred. Investigators were trying to determine whether the Munich shooter did indeed try to lure people to the place where he killed nine people and injured 27 others.

The gunman also appears to have bought his gun, a 9mm Glock 17 pistol, from the dark web; the serial number had been scratched off. He also had around 300 rounds of ammunition in his backpack, according to Munich police.

Investigators have been speaking to the gunman’s parents to gain more information, although his parents were reportedly too shocked over their son’s rampage to help investigators at first.

The 18-year-old German-Iranian did not appear to be influenced religiously or by ISIS, but he was likely mentally unstable and had an interest in rampage killings. Among his belongings, investigators found a lot of literature about rampage killings, including a book titled Rampage in My Mind — Why Students Kill.

“We know that in the apartment of the perpetrator, material was found which leads to him being interested in those who go on a rampage,” Thomas de Maiziere said. “We do not have any indications that would lead to international terrorism.”

The young man had previously needed medical treatment for mental health issues, a police official offered. He had also been the victim of “bodily injury,” which involved some other young people in an incident in 2012. Whether this helped to influence his decision to target people at the McDonald’s is still unclear. The shooter did seem to target teenagers in the McDonald’s, although not all of the victims were teens.

Flowers and other memorial items are placed near the scene of the shooting which took nine lives. (AP Photo/Kerstin Joensson)

A witness to the attack said, “I hear like an alarm and boom, boom, boom… and he’s still killing the children. The children were sitting to eat. They can’t run.”

The same witness claims she heard the Munich gunman yell “Allahu Akbar,” or “God is great,” in Arabic.

After the shooting, the gunman fatally shot himself after engaging in a shootout with police. He did not leave a suicide note at home, only lots of questions, fear, and heartbreak.

[Photo by Sebastian Widmann/AP Images]