More than 90% of survivors of the 9/11 terror attacks on the World Trade Centre in New York delayed evacuating the buildings in order to carry out tasks such as saving their work, shutting down computers, changing shoes and visiting the bathroom, according to research released today.

Interviews with 271 survivors who worked in the twin towers found that only 8.6% fled as soon as the alarm was raised. The vast majority (91.4%) stayed behind waiting for information or carrying out at least one additional task, including phoning their family and collecting belongings.

The majority put their escape back by around eight minutes, with some delayed by half an hour. People who tried to find out what was going on took between 1.5 and 2.6 times longer to respond to the alarm than those who didn't.

Researchers from the Universities of Greenwich, Ulster and Liverpool spent three and a half years interviewing former World Trade Centre workers about the day of the attack. The interviews generated 6,000 pages of first-hand accounts of what it was like trying to leave the buildings.

The report, released two days before the seventh anniversary of the atrocity, detailed how congestion on the stairs was the main cause of delay, despite the fact the towers were less than one-third full at the time of the attacks.

Modeling carried out as part of the study found that 7,592 people would have died had the buildings been at occupational capacity - more than five times the recorded death total of 1,462.

Detailing the journey down the stairs, one survivor told researchers: "Other people were coming in ... the stairwell from whatever floor they had come from ... [the travel speed] slowed down dramatically. We stopped at [floor] 55, right there, because there was obviously a lot more people. I mean we were running down for the first five stairs, 'boom, boom, boom, boom, boom', two stairs at a time sometimes. When we got to 55, we couldn't do that because we would plough into people."

Of those interviewed, 82% said they stopped at least once during their descent; a small number stopped more than 20 times. Congestion was the primary cause of stopping; only a minority of evacuees said they needed to take a rest or were stopped by environmental conditions such as debris, smoke, heat or water on the stairs.

Professor Ed Galea, project director at the University of Greenwich, said it was important to understand what factors contributed to delaying people in emergencies, in order to reduce response times. Providing people with good information about what was happening could significantly reduce response times, leading to a safer evacuation, he said.

"Together these personal stories paint a comprehensive picture of what happened and why - what influenced evacuees' behaviour. What was going through their minds when they make key decisions. This is a hugely important body of data in itself.

"We need to train people that when you hear the alarm, you need to get out. It is not important to shut down the computer or save documents," Galea said.

The answers have been put into the universities' high-rise evacuation evaluation database, which is aimed at improving the safety of tall buildings across the world.

The researchers donated $5,420 (£3,081) to the World Trade Centre Survivors' Network in recognition of the help given by participants to the study.