LOCKHART, Texas - (AP) -- 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.

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, said NTSB member Robert Sumwalt during a news conference Sunday in Washington. He offered few details of what investigators had learned so far, but said they'd 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.

A small team is already at the crash site near Lockhart, which is about 30 miles south of Austin, and Sumwalt said a larger team of investigators is headed there Sunday. NTSB member Erik Grosof said Saturday that it "will be a difficult site for us to work through."

Saturday's crash happened at about 7:40 a.m. in a pasture. Cutting through that farmland is a row of massive high-capacity electrical transmission lines about 4 to 5 stories tall. The site of the crash appears to be right below the overhead lines, though authorities haven't provided further details about what happened. Aerial photos showed an area of charred pasture underneath power lines.

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.

The balloon was operated by Heart of Texas Hot Air Balloon Rides, according to two officials familiar with the investigation. The officials spoke on condition that they not be named because they weren't authorized to speak publicly.

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.

The operation does not appear to be registered with the state of Texas.

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. The FAA's Huerta responded that regulations were unnecessary because the risks were too low.

After Huerta's reply, 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.

Speaking to the AP just before leaving for Texas to lead the crash investigation, NTSB board member Robert Sumwalt said 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.

He also noted that the team was still trying to gather basic information about the accident.

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."

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Associated Press writers Jamie Stengle in Dallas, Emily Schmall in Fort Worth, Texas, and Chad Day and Joan Lowy in Washington contributed to this report.

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Follow Joan Lowy at twitter.com/AP_Joan_Lowy. Her work can be found at http://bigstory.ap.org/content/joan-lowy