The commission investigating the 11 September 2001 attacks has said there was a breakdown in communications between the emergency services.
Footage of the attacks was shown during the hearing
Firefighters and police officers were praised for their bravery, but rivalry between the two departments hampered rescue efforts, the report found.
It was issued at a two-day hearing on the emergency response, in New York.
Film of the two planes hitting the towers was shown on big screens, as well as footage of rescuers arriving.
Testimony from rescue workers and civilians in the twin towers was also shown.
The BBC's Emma Simpson in New York says some relatives fought back tears as they revisited the chaos and confusion of that day.
After months of looking into whether government failures allowed the attacks to occur, the commission is now examining how police and fire services reacted.
Commission chairman Thomas Kean and his vice-chairman Lee Hamilton said there was some "confusion" and "lack of communication" in the emergency response.
New York's emergency services came under the spotlight
On the day of the attacks, neither the police nor the fire agency "had demonstrated the readiness to respond to an incident commander" from another department, the report said.
"This rivalry has been acknowledged by every witness we
have asked about it," the report added.
A warning from a police helicopter that the north tower was about to collapse never reached firefighters in the building.
Emergency telephone operators also had a "lack of awareness" about what was happening and were overwhelmed with the volume of calls.
Our correspondent says evacuation procedures were another critical issue.
One survivor spoke about the early public address announcements urging people to stay in the south tower after the north tower had been hit.
Those testifying at the hearings include current and
former fire, police and emergency officials.
Former New York Mayor Rudolph Giuliani is among witnesses called to testify and US Secretary of Homeland Security Tom Ridge will also appear at the hearing.
The US Congress established the bipartisan commission to establish what led to the attacks and what can be done to prevent future attacks.
It is due to deliver its final report on 26 July.