More than nine months since the New Year's Eve attacks, investigators in the western German state of North Rhine-Westphalia (NRW) are still trying to explain how hundreds of women were sexually assaulted in front of Cologne Central Station.

According to a new report, acquired by German newspaper "Express," the large groups of men - many of whom originated from the North African region - planned in advance to spend New Year's Eve on the square between Cologne's landmark cathedral and the city's train station.

Forensic psychologist Dr. Rudolf Egg was handed the task of creating the report by NRW's state parliament. Between 1997 and 2014, Egg was leader of Germany's Central Office for Criminology.

In his findings, Egg said that for the gathering to be completely unplanned, "there were too many men in the same place at the same time."

"It's conceivable this was done by word of mouth in refugee homes or other residential accommodation, or there was a plan using social media such as Facebook or WhatsApp," he added.

'Broken Windows Theory'

In the wake of Cologne's New Year's celebrations, some 1,200 criminal offenses were reported to authorities - 500 of which were sexual assaults.

One complaint read: "It was a terrible night for me. We were left with the feeling that you're worthless as a woman and that men could touch you as they liked. You felt absolutely defenseless."

According to Egg's report, 57.8 percent of victims reported a "moderate" psychological stress and 31.2 percent a "heavy" psychological strain as a result of the so-called "Sex-Mob."

The new report found, however, that the majority of perpetrators didn't go to Cologne specifically to commit sexual offenses. It was only after the men realized that police were not intervening, that the attackers felt encouraged to carry out the sexual assaults, Egg reported.

Explaining the perpetrators' motives, Egg referred to the "Broken Windows Theory," which developed in the United States. According to the theory, the inhibition to commit crimes plummets when an offense can be committed anonymously and the chance of prosecution is not expected.

"This was precisely the case in Cologne," Egg said.

The report, which is due to be officially presented to NRW's New Year's Eve investigation committee later in October, also confirmed that Cologne's police force failed to intervene when necessary.

The evacuation of the square between Cologne Cathedral and the city's central train station was carried out "significantly too late," the report said, adding that by that point, the measure could have had "no significant preventative effect."

All eyes on December 31

Initial doubts about the preparedness of Cologne's police force resulted in police chief Wolfgang Albers taking early retirement. Although never directly blaming Albers, Cologne Mayor Henriette Reker previously criticized the city's police for failing to keep her informed about the situation as it developed.

Reker, who had only been in office since December 15, said she only realized the gravity of what had happened when Chancellor Angela Merkel personally called her on January 5 to ask what her next steps would be.

With this year's New Year's celebrations now less than three months away, the pressure is on Cologne's police to prevent a repeat of last year's assaults. 

According to the local "Kölner Stadt-Anzeiger" newspaper, the head of Cologne's police force, Jürgen Mathies, is expected to announce on Thursday that there will be no fence around the city's cathedral as previously thought.

Instead, police intend to control pedestrian areas using smaller barriers, similar to those seen at sporting events. Additional lighting is also expected to be in place.

First NYE sexual assault conviction 

Several men have since been sentenced for thefts at the New Year's Eve celebrations. July, however, saw the first sentencing for sexual assault, in which a 21-year-old Iraqi was handed a one-year probation term.

According to prosecutors, the man kissed a woman against her will and licked her face after putting her in a headlock.

The second defendant, a 26-year-old Algerian, also received a one-year suspended sentence for aiding sexual assault. The Algerian also threatened the fiancé of one of the victims, asking him to "give the girls" to him or die.