"I was 100 percent sure it was a terrorist attack," said Allen Crosbie, site manager for the landscape firm Maylim Ltd., who was working at the scene.
"There was debris everywhere, a ton of black smoke. Parts of the crane, parts of the helicopter. I heard bang, bang - I presume it was the helicopter hitting the crane and then the ground. People were just panicking."
William Belsey, 25, a landscape worker, also said he heard the helicopter hit the crane.
"Luckily the crane operator was late for work this morning. He picked a good day to be late," Belsey said.
Police Commander Neil Basu said one of the dead was the pilot of the commercial helicopter, which had been flying from Redhill, south of London. No one else was thought to be aboard, Basu said; the other fatality was a person on the ground.
British aviation authorities had issued a "notice to airmen" warning pilots about the crane, which extended to 770 feet (235 meters) above ground. The crane is lit at night, and police said investigators would look at whether the light was faulty.
The area, roughly 10 blocks from the major Waterloo train and Underground station, is extremely congested during the morning rush hour. Many commuters arrive at the main line stations from London's southern suburbs and transfer to buses or trains there.
Aviation expert Chris Yates said that weather may have played a role. Investigators also would look at whether the crane had navigation lights.
"The question then becomes whether the pilot was fit," Yates said.