In a normal landing, the plane comes down on its rear landing gear first, and then the nose gear lowers onto the runway.
The finding comes after the cockpit voice recorders were recovered Tuesday afternoon, one day after the hard landing caused LaGuardia's temporary closure. Officials say the recorders were sent to the National Transportation Safety Board's lab in Washington for downloading and analysis.
The nose gear of Southwest Airlines Flight 345 arriving from Nashville, Tenn., collapsed Monday right after the plane touched down on the runway, officials said.
"When we got ready to land, we nosedived," said a passenger, Sgt. 1st Class Anniebell Hanna of the South Carolina National Guard.
Ten passengers were treated at the scene, and six were taken to a hospital with minor injuries, said Thomas Bosco, acting director of aviation for the Port Authority of New York and New Jersey, which oversees the area airports. The six crew members were taken to another hospital for observation.
Bosco said there was no advance warning of any possible problem before the landing.
He said the nose gear of the plane collapsed when it landed at 5:40 p.m., and "the aircraft skidded down the runway on its nose and then veered off and came to rest in the grass area."
He said the collapse closed the airport for more than an hour. Both of the airport's runways were back in use by Tuesday morning, a Port Authority spokesman said, and the plane was being moved to a hangar Dallas-based Southwest said 150 people were on the flight, while the Port Authority said the total was 149.
The flight was delayed leaving Nashville. Hanna said passengers heard an announcement saying "something was wrong with a tire." She and some family members were coming to New York for a visit. She said that when the plane landed, she hit her head hard against the seat in front of her.
The passengers exited the plane by using chutes. They were put on a bus and taken to the terminal. Besides the NTSB, the Federal Aviation Administration is investigating.
The landing gear collapse came 16 days after Asiana Flight 214 crash-landed at San Francisco's airport, killing two Chinese teenagers; a third was killed when a fire truck ran over her while responding to the crash, authorities said. Dozens of people were injured in that landing, which involved a Boeing 777 flying from South Korea.
Longtime pilot Patrick Smith, author of "Cockpit Confidential: Everything You Need to Know About Air Travel. Questions, Answers, and Reflections" and AskthePilot.com, said landing gear issues are not high on the list of worries for pilots.
"From a pilot's perspective, this is nearly a non-issue," he said. "They make for good television, but this is far down the list of nightmares for pilots."