The units started up after a power cut, sending powerful vibrations throughout Rana Plaza, said Main Uddin Khandaker, head of a government inquiry team.
"Together with the vibration of thousands of sewing machines, they triggered the collapse," he told AFP.
At least 500 people are known to have died and scores are unaccounted for.
Bangladesh's worst industrial disaster has sparked widespread anger in the country.
Mr Khandaker told reporters that 15 minutes before the Rana Plaza collapsed in the Dhaka suburb of Savar on 24 April, it was affected by a power cut. This set the building's generators in motion, shaking the structure, he said.
The building was not able to cope with the vibrations because it had been constructed with weak materials, such as sub-standard steel rods, Mr Khandaker added.
Arrests Late on Thursday, Bangladeshi police detained an engineer, the ninth person to be taken into custody in connection with the disaster.
Abdur Razzak Khan acted as a consultant for Rana Plaza owner Mohammed Sohel Rana, who is accused of illegally adding more floors to the building.
Correspondents say the arrest came as a surprise, as Mr Khan had reportedly warned that the complex was unsafe after cracks appeared in the structure the day before the collapse.
He appeared on a private TV station saying he had told the owners to evacuate the building because it was not safe, and that he had told government engineers that the building should be examined further.

On Thursday, garment factories reopened across Bangladesh for the first time since the collapse.
Factory workers have held daily protests calling for tough punishments for those responsible for the disaster, and demanding improved safety regulations.
Meanwhile, Bangladesh's Finance Minister, Abul Maal Abdul Muhith, told reporters on Friday he did not think the Rana Plaza's collapse would have a "really serious" impact on the garment industry.
Steps had taken to prevent future accidents, he added.
Prime Minister Sheikh Hasina also defended the industry's safety record in an interview with CNN on Thursday.
"Anywhere in the world, any accident can take place," she said, referring to last month's deadly explosion at a fertiliser plant in the US state of Texas.
Bangladesh has one of the largest garment industries in the world, with factories supplying Western retailers such as Primark in the UK and Italy's United Colors of Benetton.
The EU has said it is considering "appropriate action" to encourage an improvement in working conditions in Bangladesh's factories, including the use of its trade preference system, which gives Bangladesh duty- and quota-free access to markets in member states.