South Korean police have concluded their investigation into the crowd crush in Seoul's Itaewon district in October of last year that left 159 people dead.

The National Police Agency released a report on Friday about the situation at the scene of the crush and the causes of the tragedy.

The report says the crowd density of the people crammed into the sloping alley where many of the victims died was as high as 10.74 people per square meter.

The crowd density on a street above the slope reached 12.09 per square meter at one time.

Experts say people are in danger of suffocating when around seven are forced into one square meter.

The report says people became unable to move more than one hour before the crush.

It also says many died after they fell down and others piled on top of them.

The report cites accounts by survivors who said their feet were not touching the ground and that they were pushed to and fro and carried to the crush site.

Investigators have arrested six people, including the then-chiefs of the police station responsible for Itaewon and the local ward.