All 16 people aboard the balloon -- 15 passengers and the pilot -- were killed in Saturday's crash.
It's not clear what part of the balloon hit the lines, spokesman Robert Sumwalt said. It's also unclear whether the fire that broke out on the balloon happened before or after the collision, Sumwalt said.
The investigation will look at three main factors to determine the cause of the crash: the balloon, its operators and the environment, Sumwalt said.
He said investigators are "trying to nail down as best we can" whether fog was a factor. Though it was foggy after the accident, Sumwalt said they don't yet know whether there was fog at the time of the crash.
Friends of some of the victims identified them Sunday. They include the pilot, Alfred "Skip" Nichols, and newlyweds Matt and Sunday Rowan.
The Rowans got married in February, said Brent Jones, the father of Sunday's 5-year-old son, Jett. "Sunday was a very social person," Jones said. "They have hundreds and hundreds of friends."
Sunday Rowan bought the balloon flight for her husband as a birthday gift last year and it had taken them a while to schedule it, Jones said.
"Sunday was messaging her mom before getting on the balloon. Soon after takeoff, she stopped all communication," he said.
"It's hard, but I want everyone to understand how great our lives were together and how amazing these two people are."
Alan Lirette, the ground crew supervisor for the balloon operator, Heart of Texas Hot Air Balloon Rides, described Nichols as his "best friend, boss and roommate."
Philip Bryant, a balloon pilot, told CNN he knew Nichols.
"I knew him to be a safe, competent pilot," Bryant said. "He has done this for a very long time."
Joe and Tresa Shafer Owens were also killed, said family friend Tricia James, who set up a GoFundMe page
to raise money for their funeral.
James told CNN she worked with Tresa Owens for more than two decades at a preschool in Katy, about two hours away from where the balloon trip began.
"They adored their children and grandchildren, and loved nothing more than spending time with them," James wrote on the fundraising site.
"Joe worked hard (to) provide for his family and was always willing to lend an extra hand to anyone in need. Tresa had a long-time career at TigerLand Preschool and made an amazing difference in the lives of the hundreds of children she cared for, their families and also her co-workers and friends.
"So quick to offer up a prayer or a friendly greeting, they will forever be missed by the people who loved them," James wrote.
The NTSB said the identities of all the victims would be made public later by local authorities.
Electronic devices recovered
The FBI has found 14 personal electronic devices from those aboard the balloon, Sumwalt said. They include cell phones, one iPad, and three cameras.
The cameras, he said, are destroyed, but he said he hopes NTSB lab technicians can recover the images.
"They have been able to do miraculous things" in the past to recover information from destroyed and damaged devices, Sumwalt said.
Nichols used an iPad to navigate and a cell phone to communicate with the ground crew.
Some of those aboard posted Twitter images that investigators will examine as well, Sumwalt said.
A sunrise trip
The passengers assembled for the ride at 5:45 a.m. in a Walmart parking lot and rode in a van to the launch site, Sumwalt said.
The planned launch time for the one-hour flight was at sunrise, at 6:49 a.m., but Sumwalt said for some reason it was delayed.
"We will interview the ground crew (Monday) to see if we can find out if there was any particular reason why the launch was delayed by as much as 20 minutes," he said. "We want to find out if there was anything going on there."
The utility company responsible for the lines reported they were tripped at 7:42 a.m. The first 911 call came in approximately one minute later, Sumwalt said.
The gondola, the basket that carries passengers, came to rest below the power lines, in a field, he said. The actual balloon, called an envelope, landed 3/4 mile away.
Clashes over regulations
Saturday's crash is the most fatal hot air balloon crash in U.S. history, according to NTSB figures.
Previously, the country's deadliest hot air balloon crash was a 1993 accident in Colorado that killed six people.
In 2013, 19 people died in a hot air balloon crash in Egypt
, near the ancient city of Luxor. That was the world's deadliest hot air balloon accident in at least 20 years.
The NTSB has expressed concern about safety regulations in hot air balloon activities, but the Federal Aviation Administration, which sets regulations for manned balloons, hasn't acted on the NTSB's recommendations, said Deborah Hersman, the board's former chairwoman.
She said she urged the FAA in a 2014 letter to address "operational deficiencies" in the industry after several incidents resulted in injuries and one death.
Hersman cited accidents in 2007, 2008 and 2013, according to the letter, which also urged more stringent regulations and oversight.
"The FAA has not responded in an affirmative way," she told CNN's Sara Ganim on Saturday. "The NTSB (has) classified that recommendation 'open-unacceptable,' which means they really haven't done what the NTSB asked."
Hersman added, "Unfortunately in aviation, they say that the lessons are often written in blood, and it takes more than one event for an action to occur."
Given that some balloon gondolas can carry more than 20 people per flight, safety deficiencies mean a single accident could result in a "high number of fatalities," Hersman said in the 2014 letter, referring to the 2013 accident in Egypt.
In the letter, Hersman recommended requiring commercial balloon operators to have letters of authorization to hold tour flights and to give passengers a similar level of safety oversight as those on airplane and helicopter tours.
The FAA responded to the NTSB request last year, saying the proposed letters of authorization "would not result in a significantly higher level of operational safety."
The NTSB fired back in a 2016 statement, saying the FAA's reply was unacceptable.
It is unclear whether Heart of Texas Hot Air Balloon Rides implemented any of the NTSB's recommended measures.
Twenty-five balloon accidents, resulting in four fatalities and 25 serious injuries, have occurred since Hersman's 2014 letter and the last warning issued by the NTSB in March 2016, according to the exchange posted on its website.
The number does not take into account the fatalities from Saturday's crash.