The NTSB investigators were going over maintenance records and had talked to witnesses.
"They saw the aircraft, they heard the aircraft, and then they saw a flash of light when it hit the pole and then it crashed into the roadway,'' investigator Ralph Hicks said.
Records with the Federal Aviation Administration showed the helicopter was a Hughes OH-6A manufactured in 1967. The Hughes has historically been a military workhorse.
Atlanta police referred questions about what caused the crash to federal investigators.
Bystander Darryl James, 42, said he had gone with a companion to a check-cashing store Saturday night when he heard the helicopter flying overhead and thought it was rather low.
"The tail end went down and then there was an explosion,'' James said. He said he tried to get close to the wreckage. "As soon as I got close enough to it, poom! It exploded.''
James said people are often waiting at a normally busy bus stop near the crash site, adding, "Thank God nobody was out there.''
After the crash, fire trucks and police cars with lights flashing converged on the area, blocking surrounding roads. Police put up yellow crime scene tape and kept bystanders so far back they were unable to see the crash site behind a small rise.
Atlanta Mayor Kasim Reed said he was saddened by what he called a "terrible accident.''
"We mourn these two brave men and offer our deepest condolences to their families and loved ones,'' Reed said in a statement.