The broad outlines of what happened were heartbreaking.

An off-duty FBI agent, driving at night on a highway north of Washington, veered left and crashed into a concrete median. Another off-duty law enforcement officer, by chance on the same road, stopped to help. Both got out of their vehicles and were hit on the road shoulder by another car, their bodies hurled over the median into the opposing lanes, where one was hit again by yet another car.

Arson investigator Sander Cohen, 33, was dead at the scene, his massive injuries beyond the aid of Montgomery County firefighters and medics who arrived unaware that the victim was their close friend.

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FBI agent Carlos Wolff, 36, a husband and father of two young children, barely made it to the hospital before he died.

Details of how Wolff crashed his personal vehicle on Interstate 270 near Falls Road on Dec. 8 had not been made public pending a police crash investigation.

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The crash investigation results, obtained by The Washington Post under a public-records request, show that Wolff, alone and sober, became distracted after reaching for his cellphone at about 10 p.m. State police investigators were unable to learn why Wolff had reached for the phone.

The move, as risky as it is routine among motorists, caused Wolff’s Acura SUV to skid across at least three southbound lanes, hit the wall at the median and bounce back until it stopped, still partially in the far-left, fast lane.

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Part of the rear bumper was damaged, an indication that the SUV may have rotated before an impact severe enough to deploy the air bags.

Cohen pulled up in his personal car, a Volkswagen Jetta, parked it behind the SUV in the fast lane and turned on his hazard lights to warn other drivers.

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Cohen and Wolff, state officials say, chatted for mere minutes near the median while waiting for help.

While they stood, a 2000 Honda Accord carrying three people heading for a club in Washington veered into them.

None of the four drivers in the incident was found to be drunk, high or speeding, state police wrote.

But, state police said, the Honda’s driver, Roberto A. Garza Palacios, should have been able to more quickly spot and avoid the cars halted ahead of him, especially since Cohen had stopped and made the scene even more visible with his flashers.

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In a statement to investigators, Garza Palacios said that at the moment when he noticed a car stopped in his lane, he could not swerve away because a moving car was to his immediate right. So he swerved left, onto the shoulder, striking the two men.

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There was no indication that he hit the brakes before he hit the two men, according to investigators. A subsequent mechanical evaluation of his Honda found faulty brakes, according to state police records.

On April 11, after consulting with prosecutors at the Montgomery County State’s Attorney’s Office, state police cited Garza Palacios for negligent driving in a “careless and imprudent manner,” according to court records. The offense carries a $280 fine, three points on a driver’s record and no jail time. Its filing reflected a judgment that while driving improperly, Garza Palacios was neither “criminally negligent,” meaning he would have had to demonstrate “gross deviation” from careful driving, nor “grossly negligent,” meaning he showed a “reckless disregard” for human life — the conditions needed to support more serious charges in Maryland.

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Garza Palacios faces a traffic court trial on July 12, according to Montgomery District Court records. His lawyer, Asim Humayun, said it is too early to know if his client will pay the fine and avoid the trial, or contest the citation.

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In December, when Garza Palacios veered, he didn’t know that two men were standing in the shoulder, Humayun said. The Honda hit the median and the men, and it was only after his car came to a rest south of the impact that Garza Palacios began to realize what had happened, Humayun said.

“It was an accident — a really, really unfortunate accident,” Humayun said. “Roberto knows the reality of what happened, and he is absolutely devastated.”

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In 2015, according to court records, Garza Palacios pleaded guilty in separate traffic cases to drunken driving and negligent driving. He was issued fines but not jail time, court files show.

That year, immigration officials also sought to detain the Guatemalan native, according to Montgomery County jail records.

Garza Palacios did not return messages seeking an interview that were left at his home in Gaithersburg and with the construction company where he works.

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The northbound driver who hit Cohen after he and Wolff were propelled over the median into oncoming traffic did nothing wrong, the state police report states, describing her driving at a legal 55 mph when she suddenly saw a body horizontal across the lane she was in, three feet ahead of her Acura sedan.

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“Could this accident have been avoided? How?” she was asked last year by investigators in a written questionnaire.

“If I were in a different lane,” she wrote.

Less than four hours after the collisions, at about 2 a.m. on Dec. 9, law enforcement officers arrived at the Rockville home of Neil and Arlene Cohen to tell them that their only child had been killed.

Neil Cohen said Monday that he remembers his mind going blank and his body suddenly and involuntarily walking in tiny circles in the front hallway.

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“It’s the loss of a future,” he said, choking up at the memory of the moment that has extended to his every waking hour.

His son was a cheerful child who visited a fire station as a teenager, the father recalled, seeding the notion of the career he would pursue. Sander Cohen became a volunteer firefighter in Rockville and eventually joined the Office of the State Fire Marshal, which handles fire and arson investigations for many counties.

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“He had that great smile,” Neil Cohen said. “He would always help people.”

Neil Cohen said he did not fault Wolff for reaching for his phone, saying it did not cause his son’s death. He questioned why state troopers didn’t force Garza Palacios to be tested for alcohol impairment.

Police in Maryland can order testing after crashes involving death or life-threatening injury, but only when an officer has “reasonable grounds” to suspect that the driver was impaired by alcohol.

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In the I-270 collision, state troopers “observed nothing that established those reasonable grounds to order a test,” said Greg Shipley, a state police spokesman.

Wolff and his family moved from Venezuela to Montgomery County when he was 8 years old. He went on to graduate from American University and had been with the FBI for 11 years, most recently as a supervisory special agent at headquarters in Washington.

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On Monday, Marla Wolff, Carlos’s widow, said their daughter just turned 3 and knows that her father is no longer here, even as she seems to have a peace about her. Their son, who is 7, knows what happened. “I miss Daddy so much,” he says, sometimes sobbing.

Marla Wolff said she has never understood how Garza Palacios did not have enough time to slow down and avoid the crash, as Cohen had been able to do. And like other members of Carlos’s family, she questioned how, given his previous driving record, Garza Palacios was allowed to be in the road.

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That Wolff was grasping for a phone hardly made him unique on the nation’s roadways.

In the Washington region, at least 2 million drivers reach for their phones every day, said John B. Townsend II, a spokesman for AAA’s Mid-Atlantic region.

“We know it’s widespread. We know it’s pernicious,” Townsend said. “We see it. We do it. It’s become a pandemic on our roads.”

State police investigators learned that Wolff had reached for a phone after they talked to a person he called after he crashed, who related that Wolff had said that reaching for the phone led to the crash.

Garza Palacios has had previous traffic and criminal violations.

On May 17, 2015, in Gaithersburg, Garza Palacios was cited for negligent driving and not following traffic control signs. Twelve days later, he got out of his car after driving onto a sidewalk, where he then smashed several beer bottles and began slamming a construction tool into a light pole, police said in court filings. The filings said his blood alcohol level was 0.13, nearly two times the legal limit for driving. He was cited for drunken driving and numerous other offenses.

Within the next week, Garza Palacios allegedly used a steel bar to bash a window of a Dunkin’ Donuts and the windows of about 16 cars and set a sofa on fire near a construction site, according to police allegations.

In the driving cases, Garza Palacios pleaded guilty to negligent driving, not following traffic control signs and driving while impaired. He was fined a total of $330 and did not receive jail time. On the other matters, he served about four months in jail, according to court records.

His charges drew the attention of federal immigration officials, who on Aug. 3, 2015, requested that Montgomery County jail officials hold Garza Palacios on a detainer so federal officials could assess whether to deport him.

The request to hold him, jail officials said, was not honored because the county then had a policy to execute such holds only for requests made through an arrest warrant or with probable cause cited by immigration agents. Garza Palacios was released and went to work at the construction company.

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