AMSTERDAM -- Church services were held Sunday in a quiet suburb outside Amsterdam to mourn six people killed by a gunman at a crowded mall, as investigators puzzled over his motive and struggled to explain how he was able to obtain and legally own five firearms in the Netherlands.

The attacker, identified as 24-year-old Tristan van der Vlis, had minor run-ins with the police, including an illegal weapons possession charge in 2003 when he was 17 that was eventually dropped, District Attorney Kitty Nooy said.

Prime Minister Mark Rutte (C) and the Minister of Justice Ivo Opstelten (2nd R) look at the candles lit at the Ridderhof shopping centre in Alphen aan de Rijn April 10, 2011. [Photo/Agencies]

"This is definitely part of our investigation," she said.

Van der Vlis opened fire in Alphen aan den Rijn with what witnesses described as a machine gun on one of the first pleasant Saturdays of spring, authorities said. In addition to the fatalities, he wounded 15 others, including an infant, before fatally shooting himself in the head at the Ridderhof mall, officials said.

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Three of the wounded remained in critical condition. The infant was among two people slightly wounded, but was under observation at a hospital. Authorities refused to say whether any other children were killed or wounded in the attack and they haven't released a list with the identities of the victims, citing privacy reasons.

Police said they discovered a note from Van der Vlis in his car, threatening bombings at other malls. His mother found a suicide note at the home where he lived with his father near the mall. Neither gave any indication of why he wanted to hurt other people.

Alphen is about 20 miles (30 kilometers) southwest of Amsterdam. Church services closed to the media were being held at the Goede Herderkerk (Good Shepherd Church). A church closer to the mall is cordoned off in connection with the ongoing forensic investigation into the shooting.

"This hacks our society apart," Mayor Bas Eenhoorn said at a news conference. "I hope that people seek each other out to express their grief and their fears that are the result of this terrible incident."

A candlelight procession to the mall was planned for Sunday evening.

Van der Vlis's apartment building remained under police guard. Neighbors gave mixed comments about his character.

"He seemed to me to be a nice guy, he always greeted me nicely," said Veronique Troon. But she said that one time he asked her about her native Brazil, saying "'that seems like a very, very dangerous country, don't you think so?' I found it very weird."

An online condolence register has been signed 5,000 times.

"I always thought these terrible things only happen abroad, but now here too," wrote Thea Hilbrants of Groningen, Netherlands. "Terrible, incomprehensible. I wish much strength for everyone touched by this."

Queen Beatrix and Prime Minister have also expressed their shock and sympathy for victims.

Witnesses, including some interviewed by The Associated Press, said the man stalked the halls of the mall, shooting in an attack that lasted 10 minutes or more. Most witnesses have said that Van der Vlis was shooting with a machine gun, but the type of weapon has not been identified by authorities.

Witnesses said he later shot himself in the head with a revolver.

Nooy told reporters Saturday the first indications were that Van der Vlis had permits for five guns, including three that he carried during the rampage, according to a preliminary investigation.

Under Dutch law, owning revolvers and rifles is only allowed with a permit, and they must be kept locked up unless they are in use at a firing range or during hunting. Automatic firearms are legal only for use by the military and special police units - hough an explanation of the law on the Justice Ministry website mentions there are other exemptions under some circumstances, for collectors for instance.

Automatic weapons are frequently found during drugs busts. Investigators have said Van der Vlis had no military background.