At least 16 people thought to be on a hot air balloon that crashed in Central Texas were dead, the National Transportation Board said Sunday, noting that investigators were still trying to figure out exactly how many people were on the balloon and what caused the tragedy.

NTSB member Robert Sumwalt said Sunday that a full team of investigators would soon be on the ground near the town of Lockhart, where a hot air balloon plummeted to the ground in a pasture below power lines Saturday.

Officials originally said that everyone who was riding in the balloon appeared to be dead, but they couldn’t confirm the exact toll. Sumwalt said Sunday that at least 16 people had died, but because air balloon operators don’t often file a manifest, it’s hard to know how many people were in the aircraft.

Related: Texas Hot Air Balloon Crash: How Safe is the Aerial Activity?

Sumwalt said investigators would be looking at three things, as is often the case in aerial accidents: the human, the machine and the environment.

Investigators would also be gathering cell phone video from witnesses and anything that could be recovered from victims’ phones, Sumwalt said.

One witness, Margaret Wylie, who lives near the site of the crash about 30 miles south of Austin, told NBC News on Saturday that she heard a pop, and when she looked up “there was a big fireball.”

“I think the pops I heard was the balloon connecting with the power lines,” Wylie said.

Still, Chris O’Neil, an NTSB spokesman, told NBC News that it was “far too early to speculate about cause.”

“We go through our investigative process without any preconceived notions,” O’Neil said. “The facts lead us where they lead us.”

Gathering other evidence would be a sort of race against time he said, since investigators were delayed by weather in getting to the site, and most physical evidence will be tarnished within days, Sumwalt said.

“We’re looking at operation of balloon, pilot, and company that operated the balloon,” Sumwalt said.

Officials have not identified the pilot of the doomed balloon or the company that operated it, but the owner of Heart of Texas Hot Air Balloon Rides’ Facebook page told NBC News that they owned and flew the balloon.

Officials also have not identified any victims, but the brother of a 34-year-old and his wife said they were on the balloon. Joshua Rowan told NBC News that his brother, Matt Rowan, 34, and his sister-in-law, Sunday Rowan were on the doomed flight and would be “incredibly missed.”

Image: Sunday and Matt Rowan, both 34, were killed in Saturday's balloon accident in Texas

Image: Sunday and Matt Rowan, both 34, were killed in Saturday's balloon accident in Texas

The couple had been recently married, and Matt Rowan had just started a new job as an army hospital burns trial unit chief. “He was doing some amazing work and research. He felt like a lot of the stuff he was doing would have benefits for soldier and other service members who had been injured by burns,” Joshua Rowan said.

The couple from San Antonio “was so happy together,” Rowan said. “They were trying to a grow their family. It makes the timing of it even more horrific.”

The balloon accident marks one of the worst in the world. In February 2013, a balloon caught fire while floating over Luxor, Egypt, killing 19 of the 21 on board.

In April 2014, the NTSB recommended that the Federal Aviation Administration bolster ballooning regulations in the U.S., warning of “the potential for a high number of fatalities in a single air tour balloon accident.”

The FAA rejected the suggestions, saying ballooning risks were low.

A month later, three people were killed when a hot air balloon hit a power line, exploded and crashed in eastern Virginia.

Lynn Lunsford, a spokeswoman for the FAA, told NBC News on Sunday that it was “too early to say” whether the FAA would reconsider the NTSB recommendations “until we’ve had a chance to gather and examine the evidence in this particular case.”