WASHINGTON — A tugboat officer, focused on his cellphone and laptop, allowed his tug to push a 250-foot barge into a stranded tourist vessel on the Delaware River in July, killing two people and highlighting a growing problem with distracted operators on land, sea and air, the National Transportation Safety Board said Tuesday.

“This accident is not just about one individual’s actions, but about a new and highly troubling societal norm,” Chairwoman Deborah Hersman said as the safety board conducted its final hearing on the crash.

“When people’s lives are in your hands, whether you’re piloting a tug, conducting a train, flying a 757, or even driving home this evening, you take responsibility by giving your full attention to the safety-critical task at hand,” Hersman said. “There is simply no conversation or action that is important enough to risk your life or the lives of others.”

The board found the probable cause of the accident was the frequent use of a cellphone and computer by the tugboat’s first mate, Matt Devlin, who was responsible for guiding the vessel. He was also faulted for navigating from the lower wheelhouse, where visibility was reduced.

The first mate’s attention to a family medical crisis caused him to overlook the duck boat stranded in the river, investigators found.

A deckhand on the duck boat also was distracted because he was texting on his cellphone in the minutes before the accident, investigators said.

Both men were breaking their companies’ rules against such use of electronic devices.

Too many other accidents recently investigated by the NTSB have similarly been caused by distracted operators, Hersman said.

“We must find a way to change the culture of distraction we see across transportation because, frankly, the distractions are only going to get worse,” she said.

The July 7 collision killed two Hungarian tourists, Dora Schwendtner, 16, and Szabolcs Prem, 20.

Devlin was the first mate of the tug Caribbean Sea, which was pushing a 250-foot sludge barge on the Delaware. He was responsible for watching out for river traffic in front of the barge.

The five-member safety board convened Tuesday morning to adopt the findings, probable cause and safety recommendations from the final report presented to the board by NTSB investigators at the hearing.

Devlin initially told his bosses and the Coast Guard after the accident that he had been very concerned about his 6-year-old son, who had nearly died during relatively routine surgery. Devlin refused to talk to investigators after those initial interviews.

Investigators said Tuesday that Devlin “failed to perform the fundamental tasks to maintain a proper lookout.” They said he apparently left the tugboat’s upper wheelhouse, where visibility was much better, to go to a lower vantage point as he continued to use his cellphone and the company laptop.

Devlin made 15 calls and received six between noon and the 2:37 p.m. crash, investigators said. He also used the computer to look up medical information, they said.