PARIS, France (CNN) -- The Air France plane that crashed a month ago off the coast of Brazil "did not break up or become destroyed in flight," but bellyflopped intact into the Atlantic Ocean, the French air investigation agency announced Thursday.
"The plane went straight down, almost vertically... towards the surface of the water, very very fast," air accident investigator Alain Bouillard said.
Based on visual study of the physical remains of the Airbus A330 that have been recovered, "we were able to see that the plane hit the surface of the water flat. Therefore everything was pushed upwards -- everything was pushed from the bottom to the top" of the plane, he said.
The 228 people killed in the crash "had no time to prepare," he said.
But Bouillard said he did not have autopsy results from the bodies recovered, and did not know why no one lived through the crash.
"I don't know why nobody survived," he said. "I don't know the intensity of the impact. Perhaps we will find out from the autopsies. Perhaps we will never know."
Bouillard said it was still unclear what caused the crash, the deadliest in Air France's 75-year history.
"Today we are very far from establishing the causes of the accident," he said.
But there is no reason to ground Airbus A330 airplanes, he said. "There is no problem with flying these airplanes."
Pressed by a reporter on why he was not ordering the model to stop flying, he said the fleet has flown millions of miles and there are currently 660 of them flying. "Statistically, this would answer the question," he said.
Air France 447 was unable to fly on autopilot at the time of the crash, the investigator said. That was because the autopilot was not receiving speed, wind or direction information, he said.
"These tell us that the plane has to be, in this case, directed by the pilot," he said. He did not immediately say if the pilots were in control of Air France 447.
The last contact with the plane was at 2:10 a.m. local time on June 1.
"Right after that 24 automated messages came through" about the status of the plane, he said. Those messages were what enabled investigators to determine that the autopilot would not have had enough information to fly the plane, he said.
No air-traffic controllers seem to have been monitoring the flight when it went down, investigations have found. It would normally have been "handed over" from controllers in South America to others in Africa while flying over the Atlantic, but that did not happen, Bouillard said.
The plane was flying through severe storms when it went down.
Three other flights on similar routes changed course within an hour after Air France 447 flew into the bad weather.
A Spanish, French and German flight all experienced turbulence in the same region and diverted as much as 100 km (62 miles) off course to avoid bad weather, Bouillard said. All three flights had problems communicating with air traffic control.
Investigators will continue searching for the flight data recorder and the cockpit voice recorder -- commonly known as "black boxes" -- until July 10, said Bouillard, of the French air accident investigation agency known as the BEA.
"They normally give a signal for 30 days. We will keep listening another 10 days," he said.
Air France said Thursday it was of "capital importance" to find the recorders, "which would enable the investigators to analyze the causes of the accident, whatever these may be. No effort must be spared in achieving this end."
Bouillard said investigators would continue to search even after the beacons on the recorders stop signaling, in what he called a "second phase" of the search.
"If we could find a part of the plane that we know was near the black boxes, that will give us a clue about where to search," he said.
The mountainous ocean floor in the search area ranges from 3,280 to 15,091 feet, BEA officials have said in the past, making the search for the recorders -- and the rest of the plane's debris -- difficult.
"It is as if it fell in the Andes," Olivier Ferrante, chief of the BEA search mission said last month.
French submarines and sensitive U.S. military listening devices are being used in the search.
Brazil called off the search for bodies on June 27, having found 51 of the 228 people who died when the plunged into the sea, according to the military.
Investigators have also found more than 600 parts and structural components of the plane, along with luggage, Bouillard said.
They have not found any clothing, he said, but was unable to say why.