At least 16 people died in the hot air balloon crash in Central Texas, the National Transportation Safety Board said Sunday, adding that investigators are still trying to determine the exact number of passengers and what caused the accident.

It's apparently the worst such disaster in U.S. history, and one of the worst ever in the world. In February 2013, a balloon flying over Luxor, Egypt, caught fire and plunged 1,000 feet to the ground, crashing into a sugar cane field and killing at least 19 foreign tourists.

NTSB investigators are beginning the process of determining what caused the balloon to crash Saturday morning in a pasture near Lockhart, Texas, which is about 30 miles south of Austin, NTSB member Robert Sumwalt said during a news conference Sunday in Washington. He noted they'll be looking at "three things -- human, machine and environment."

Investigators will scrutinize the company that operated the balloon and the pilot, neither of which have been publicly identified. The balloon was operated by Heart of Texas Hot Air Balloon Rides, according to two officials familiar with the investigation who spoke to The Associated Press on condition that they not be named because they weren't authorized to speak publicly. The operation does not appear to be registered with the state of Texas.

The NTSB also said they'll look at the aircraft's maintenance history and weather at the time of the crash.

They're not sure how many people were riding in the balloon and will look into whether the operator of the balloon filed a passenger manifest before taking off, also noting that balloons do not usually file flight plans, Sumwalt said. He added that federal officials are particularly interested in any cellphone video of the balloon's flight, and that investigators will look for devices in the wreckage that have recoverable video shot by passengers, as well as any video from witnesses.

"When balloons go out on these flights, they have a chase couple of cars to go pick up the riders after they've landed in a field somewhere. We think there may be some chase footage from those cars," Sumwalt said.

The crash happened in farmland, and cutting through it is a row of massive high-capacity electrical transmission lines. The site of the crash appears to be right below the overhead lines and aerial photos showed an area of charred pasture underneath, but authorities haven't provided further details about what happened.

Margaret Wylie lives about a quarter-mile from the crash site and told The Associated Press that she was letting her dog out Saturday morning when she heard a "pop, pop, pop."

"I looked around and it was like a fireball going up," she said, noting that the fireball was under large power lines and almost high enough to reach the bottom of them.

Wylie, who called 911, said the weather seemed clear and that she frequently sees hot air balloons in the area.

Heart of Texas' website said it offers rides in the San Antonio, Houston and Austin areas. The company's Facebook page features photos of a hot air balloon with a smiley face with sunglasses on it up in the air, people waving from a large basket on the ground and group selfies taken while up in the air.

Authorities have not released the names of those who were on board and have not said who was flying the balloon.

Skip Nichols identifies himself on his Facebook page as the chief pilot of Heart of Texas and pictures posted by him are on the business' Facebook page. Nichols, 49, is also the registered owner of Missouri-based Air Balloon Sports LLC. No one answered the door at a home in Kyle, Texas, believed to be his. Calls to Heart of Texas operations manager Sarah Nichols, 72, rang unanswered, and a woman in Missouri believed to be his sister did not return calls seeking comment.

Warning about potential high-fatality accidents, safety investigators recommended two years ago that the Federal Aviation Administration impose greater oversight on commercial hot air balloon operators, government documents show. The FAA rejected those recommendations.

In a letter to FAA Administrator Michael Huerta in April 2014, the National Transportation Safety Board urged the FAA to require tour companies to get agency permission to operate, and to make balloon operators subject to FAA safety inspections.

Huerta responded that regulations were unnecessary because the risks were too low, and the NTSB classified the FAA's response to the two balloon safety recommendations as "open-unacceptable," which means the safety board was not satisfied with the FAA's response.

Sumwalt said Saturday that he was studying the board's recommendations from previous hot air balloon accidents. "I think the fact that it is open-unacceptable pretty much speaks for itself," he said.

FAA spokesman Lynn Lunsford said it's difficult to say whether the Texas crash will cause the agency to reconsider NTSB's recommendations "until we've had a chance to gather and examine the evidence in this particular case."