Police probing what led a man to ram his van into an open-air restaurant in the western city of Münster found an 18-page letter in one of his apartments, according to the Süddeutsche Zeitung daily newspaper.

In the missive, the suspect — identified as German national Jens R. — referred to "repeated nervous breakdowns" and "outbreaks of aggression," as well as a botched medical operation. He also wrote about humiliating childhood experiences, and anguish caused by serious problems with his parents.

Read more: Münster: A sleepy city of culture turns into a crime scene

Police have not confirmed the discovery of the letter, but they did reveal that an email R. wrote to several people in March contained "vague hints of suicidal thoughts, but no indication he might endanger the lives of others."

Jens R. plowed his camper van into a crowd on Saturday afternoon, killing two people and injuring more than 20 others. He then shot himself.

The interior minister for the state of North-Rhine Westphalia, where the attack happened, said on Monday that Jens R. did not have a weapons permit for the firearm he used. "The weapon was not obtained through normal channels," Herbert Reul told broadcaster WDR 5. 

Read more: Do bollards offer protection against vehicle attacks? 

Moving service

More than 1,500 people, including federal ministers and personnel from emergency services, attended a memorial service in Münster's famous cathedral on Sunday night.

Bishop Felix Genn urged mourners to try to understand the tragic events, saying that he had heard cries of helplessness and demands for an explanation.

"It's good to talk to others about this questioning. We do not have to find answers right away," he said.

A 51-year-old woman and a 65-year-old man were killed in Saturday's attack. Several people are still in critical condition.

Sole attacker

Authorities say they believe R. was the sole attacker, and that there was no indication his actions were motivated by politics or religion.

"It looks as though we're dealing with a mentally unstable and disturbed offender, who had clearly long been thinking about taking his own life," Herbert Reul, interior minister in the state of North Rhine-Westphalia, told Deutschlandfunk on Monday.

"But if someone is thinking about taking their own [life], it doesn't automatically mean that they will commit violence against other people."

The 48-year-old was known to police after being accused of various offenses in 2015 and 2016, including property damage, fraud and threatening behavior.

nm/rt (AFP, dpa, epd, Reuters)