Kincaid initially seemed fine and complained only of chest pain from the impact of his car’s air bag. But four hours later at Inova Fairfax Hospital he began suffering headaches, which led to the discovery of a subdural hematoma, or bleeding in the brain. Emergency surgery did not reduce Kincaid’s brain swelling, and he died six days later. He was 78. In July, Cadigan, then 66, was indicted on a charge of involuntary manslaughter.

Kincaid’s wife of 41 years, Rita Kincaid, grieved the loss of her husband. They had been retired for years and traveled around the world together, with another cruise planned. She followed Cadigan’s case, met with McGhie and prosecutors and attended pretrial hearings. Then last month, as the trial date approached, the prosecutors told her they were dismissing the case. Cadigan would face no charges. It was over.

“I broke down and cried,” Rita Kincaid said. “I can’t believe this. This can’t be true.”

Fairfax prosecutors said that they felt they had no choice — that they could not trust McGhie. Cadigan’s defense attorney had noticed that an audio recording of McGhie’s interactions with Cadigan did not match what he wrote in his report, and when McGhie wrote a second report, he changed the sequence of events and made them wrong. The recording also showed no sign the officer informed Cadigan he was being placed under arrest, omitting a crucial event in a drunken-driving case. Then the officer ordered Cadigan to go in an ambulance to the hospital to have his blood drawn but did not get a search warrant for the blood, said his attorney, Marina Medvin. She asked a Fairfax judge to throw out all the evidence against Cadigan.

Then, as a hearing on Medvin’s motions was about to begin, McGhie told the prosecutors that there was another recording showing he had arrested Cadigan and that events occurred as he claimed, according to a prosecutor’s letter in the case.

But there was no other video. And Assistant Commonwealth’s Attorney Bridget Corridon was forced to explain to Rita Kincaid that she could not ethically put McGhie on the witness stand if she could not believe him, leaving prosecutors with no case. The drunken-driving charge against Cadigan had already been dismissed, replaced by a manslaughter indictment. Now the manslaughter charge was gone as well.

Both the Fairfax prosecutors and Rita Kincaid said they filed complaints against the officer last month, and Fairfax police said they have launched an internal-affairs investigation of McGhie, 47, an 11-year member of the Fairfax force who works as a patrol officer in the West Springfield District. Lt. Steve Wallace said that because an administrative investigation was pending, the department would not comment on the case. McGhie did not return a call and email seeking comment. He remains on active duty with pay, Wallace said.

“I always take incidents that result in the loss of life very seriously,” said Fairfax County Commonwealth’s Attorney Steve Descano, who took office this month. “Although this happened before I took office, I have reviewed the details of this particular case and am confident that our office acted in accordance with our ethical requirements.”

Rita Kincaid’s attorney, Lawson Spivey, said, “I just hope that someone in the commonwealth’s attorney’s office can take a look at this case and give us some amount of justice for William.”

Rita Kincaid added: “And justice for Cadigan. He needs some justice, too.”

Descano said he had no plans to reopen the case, because “there is no clear criminal charge that can now be pursued.”

Cadigan declined to be interviewed. Medvin said Cadigan “is a 67-year-old husband, father and grandfather. He’s a good man. He was trusting of the system. He checked on the well-being of Mr. Kincaid the next day,” by calling the Kincaid residence. Cadigan and Medvin did not learn of Kincaid’s death until three months later, when prosecutors informed them they were seeking a manslaughter charge.

Rita Kincaid confirmed she received a call from Cadigan the day after the crash. She had spent the night at the hospital, after her husband had undergone emergency brain surgery. “I had just gotten home,” she said. “The phone rang. He said something about being sorry. I said, ‘You should be in the hospital, not my husband,’ ” and she said she criticized him for drinking and driving. “He told me, ‘I know I have a problem,’ ” Rita Kincaid said. “I have that conversation plastered in my brain.”

The crash had not been a particularly violent one, and damage to the cars did not appear major. Cadigan was surprised to learn William Kincaid had been hospitalized and “did not say much in response” to Rita Kincaid’s comments, Medvin said.

Cadigan and William Kincaid were driving in opposite directions on Zion Drive, a short but well-traveled two-lane road just off Route 123 south of Fairfax City, around 12:50 p.m. on Feb. 8. Kincaid was driving east toward his home, and Cadigan was heading west when he turned into Kipp Court toward his residence. McGhie recorded in his crash report that the two cars were moving only about 20 mph, though the speed limit there is 35. Since the crash did not appear to involve serious injury or a question about how it happened, McGhie did not summon specialized crash investigators to the scene.

According to his reports, McGhie was alerted by paramedics to the suspicion that Cadigan had been drinking. He spoke to Cadigan, then gave him a preliminary breath test with a portable device that is not admissible in court but that is used by officers to help determine whether someone has been drinking. Cadigan’s blood alcohol level in that device was 0.15, according to court records. McGhie wrote in his first report that he then placed Cadigan under arrest and Cadigan then admitted drinking. The officer then ordered Cadigan into an ambulance and sent him to Inova Fairfax for a blood test, without performing a field sobriety test, citing Cadigan’s “condition.”

When Medvin received the police reports in the case, she found that McGhie did not have a body camera but did have a body microphone connected to his in-car camera. But he had not turned the microphone on, and the car’s camera was not pointed toward the officer, Medvin said. Instead, another officer’s body microphone captured McGhie’s conversation with Cadigan, Medvin said, though not much video of the incident.

On the recording, Medvin heard that Cadigan repeatedly denied drinking, told the officers he was taking medications and was never told he was under arrest or informed of his Miranda rights not to incriminate himself. Cadigan asked to go home or call his wife, and McGhie told him no, Medvin said.

The lack of a formal arrest is important, Medvin said, because in Virginia drivers are considered to have given “implied consent” to alcohol testing if they drive and are arrested. Instead, the officer merely detained Cadigan for three hours to have his blood drawn without his consent, Medvin said. Absent a suspect’s consent or an emergency, the Supreme Court has ruled, police must get a search warrant to obtain someone’s blood. McGhie did not do that, either. Medvin moved to have the blood test, and its .31 result, thrown out of the case.

Medvin then saw McGhie had filed a new version of his report in August, changing the sequence of events and stating Cadigan had admitted drinking before being arrested. The second report created “a more likely scenario that would hold up in court,” Medvin said. “I guess he did not expect the system to hold both versions of events” or that another officer’s audio recording would contradict his report, she said.

Medvin reviewed the medical records and consulted experts who said William Kincaid could have died of a variety of causes, including blood-thinning medication he had been taking, prior medical conditions, a lung puncture during surgery or possibly a slow-developing head injury before or after the crash, because there was no visible injury after the crash. She moved to dismiss the case on those grounds.

With trial approaching in December, the two sides met in the Fairfax courthouse on Nov. 15 for a hearing on Medvin’s motions to throw out evidence or dismiss the case. But before the hearing, “Officer McGhie informed me that he ‘doesn’t think that the in-car video is the same,’ ” Corridon wrote in a letter to Medvin. “He stated that the first in-car video that he saw was much longer … and you could hear Officer McGhie tell the defendant he was ‘under arrest.’ ”

The hearing was delayed “to investigate these statements by Officer McGhie,” Corridon wrote. But soon, prosecutors informed Medvin there was no other video. On Dec. 5, the prosecutors dismissed the case.

“I do not understand how the law can support this,” Rita Kincaid said. “The perpetrator walks free leaving me with the loss of my husband of 41 years and no justice or accountability for the drunk driver. I’m at a loss as to how this could happen.”