PHILADELPHIA (AP) — The cockpit voice recorder was not working on a medical transport plane that killed seven people when it plummeted into a Philadelphia neighborhood in January and likely had not been functioning for several years, the National Transportation Safety Board said in a preliminary report Thursday. The NTSB also confirmed the crew made no distress calls to air traffic control. A ground warning system that may contain flight data memory is still being evaluated by the manufacturer, the agency said.

The plane plummeted into a residential and commercial area within a minute of taking off from Northeast Philadelphia Airport and erupted into a fireball on Jan. 31. Officials said the crash killed all six people aboard the Learjet 55 and a seventh person who was in a vehicle on the ground. At least two dozen others were injured, including a 10-year-old boy in a vehicle who was hit by debris while trying to protect his sister.

Former NTSB Chairman Jim Hall called the finding about the cockpit recording “disturbing” because “that and the whole flight data recorder are important to find out what went wrong.”

“It’s a significant loss of important information that should have been there,” Hall said.

He noted that the lack of any distress call shows the emergency occurred too quickly for the crew to communicate with the tower.

Those on the plane included an 11-year-old girl who had received medical treatment at Shriners Children’s Philadelphia hospital. Jet Rescue Air Ambulance said the plane was taking Valentina Guzmán Murillo and her 31-year-old mother, Lizeth Murillo Osuna, home to Mexico.

Messages seeking comment were left Thursday with Jet Rescue. The company previously identified its team aboard as Dr. Raul Meza Arredondo, 41; the captain, Alan Montoya Perales, 46; copilot Josue de Jesus Juarez Juarez, 43; and paramedic Rodrigo Lopez Padilla, 41. All four were from Mexico.

According to the report, the recorder was recovered 8 feet (2.4 meters) underground after the crash and had significant damage, including exposure to liquids. After extensive cleaning and repairs, the agency discovered the 30-minute tape didn’t have audio of the flight.

The high-impact crash destroyed or badly damaged more than a dozen homes and business, leaving debris from the plane scattered across a wide area nearly 500 yards (457 meters) long and 300 yards (274 meters) wide.

Former NTSB investigator Jeff Guzzetti said the loss of any cockpit recordings makes the agency’s work more difficult, but not impossible. He hoped the ground warning system can provide some data, and wondered why the voice recorder hadn’t been inspected regularly.

“I really think that puts a black mark on this Mexican operator, for not ensuring that their cockpit voice recorder was operating,” Guzzetti said. “The NTSB, I think, will still be able to come to a probable cause, just because they’re really good at extracting circumstantial evidence.”

Under Mexican regulations, owners are supposed to include the voice and flight data recorders in the maintenance plans for aircraft, and the government authority where aircraft are registered is responsible for supervising those plans and checking aircraft to make sure that what’s in the documents is true at least once a year, said Rogelio Rodríguez Garduño, a professor of aviation law at the National Autonomous University of Mexico.

Civil aviation authorities in Mexico have not responded to an Associated Press request for documents about Jet Rescue’s maintenance.

Guzzetti, a lead NTSB investigator on John F. Kennedy Jr.’s fatal crash near Martha’s Vineyard, believes the Philadelphia crash has some of the same hallmarks of a pilot suffering from “spatial disorientation” in dark or cloudy skies.

That occurs, he said, when pilots lose their bearings, don’t trust their instruments and turn, sometimes repeatedly, in a misguided attempt to correct course. The Learjet in Philadelphia, he noted, “came screaming out of the sky — and it did some turns too — and again you see those same types of turns in the JFK Jr. accident.”

“The human body can play tricks on you, and that’s why you have to be incredibly vigilant as a pilot and trust your instruments,” he said. “But, you know, it’s not to say that there couldn’t have been some sort of distraction in the cockpit too that occurred during that time.”

Several victims on the ground retained law firms to represent them in potential lawsuits, including a man badly burned after his SUV became engulfed in jet fuel.

The crash was among recent aviation disasters and close calls that left some people worried about the safety of flying. It happened two days after an American Airlines jet and an Army helicopter collided in midair in Washington, D.C. — the deadliest U.S. air disaster in a generation.

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Reporters Maria Verza in Mexico City and Mark Scolforo in Harrisburg, Pennsylvania, contributed to this report.