A hot air balloon that crashed in a Texas pasture, killing all 16 people on board, likely struck a power line, an investigator said on Sunday.
"There's physical evidence to indicate the balloon or some component of the ballon hit the wires themselves," US National Transportation Safety Board (NTSB) official Robert Sumwalt said at a news conference near the crash site.
A power line runs prominently across the field.
It was one of the deadliest hot air balloon crashes in history. Collisions with power lines are a leading cause of hot air balloon accidents.
The NTSB, taking the lead in investigating Saturday's crash, was asking the public for any cell phone videos of the incident.
Investigators were also hoping to retrieve evidence from 14 devices - phones, cameras and an iPad - recovered from the crash site, the Austin American-Statesman newspaper said.
Identifying the victims will be "a long process", the Caldwell County (Texas) sheriff's office said in a statement.
"Our goal is not to solve it this week," Sumwalt said.
"Our goal while being on scene is to collect the perishable evidence" and ultimately determine "why it happened so that we can keep it from happening again", he added.
CNN and local media identified the balloon's pilot as Alfred "Skip" Nichols, owner of Heart of Texas Hot Air Balloon Rides.
They said Nichols owned at least three balloons, one well-known in the area for sporting a huge yellow smiley face against a red, white and blue background.
A photo on Facebook showed him piloting a balloon with a large wicker basket holding more than a dozen passengers.
CNN quoted Alan Lirette, who worked for the company and said he had helped load the passengers on Saturday, as confirming Nichols' death and saying there were no children aboard. He said Nichols was a close friend and a "great pilot".
The Federal Aviation Administration (FAA) and the Federal Bureau of Investigation are assisting in the inquiry.
NTSB experts will first examine "the operation of the balloon, the pilot, the company that operated the balloon", Sumwalt said.
The balloon burst into flames and plummeted to earth soon after dawn outside the town of Lockhart, some 30 miles (50 kilometers) south of Austin, the FAA said in a statement.
Weather did not appear to be a problem - temperatures were moderate and wind was light.
"I didn't see the balloon hit. I just heard the popping. And I heard the popping, and then the next thing I knew is the fireball went up," Margaret Wylie, 66, who witnessed the crash, told broadcaster TWC News Austin.
The 16 deaths make the balloon accident the deadliest on record in the United States. Previously, the highest number of fatalities in a single US hot air balloon crash was six.
In 2013, a sunrise hot air balloon flight over Egypt's ancient temple city of Luxor caught fire and crashed, killing 19 tourists. The pilot and one other tourist survived by jumping from the balloon.