Photographs from FBI files of the Chevrolet Impala, which had been assigned to FBI agent Matthew Lowry, in which heroin and other evidence was found. Some areas were redacted by court officials to prevent disclosure of sensitive information. (U.S. District Court)

For months, the FBI agents who worked with Matthew Lowry at the Washington Field Office explained away his erratic behavior as stress from a new baby, a troubled marriage and the pressure of building an expensive home in Southern Maryland. The usually dependable agent had become harder to reach, particularly during his off-hours.

Concern turned to desperation the night of Sept. 29, when the 33-year-old didn’t return home after work. His father, then an assistant police chief in Anne Arundel County, called his son’s friends and colleagues, launching a frantic, hours-long search across the District that ended under a crane in a dusty construction lot across from the Navy Yard.

There, two FBI agents found Lowry standing next to his black, government-issued Chevrolet Impala, and he was incoherent. Afraid he was contemplating suicide, the agents took Lowry’s sidearm from the holster and his M4 rifle from the trunk. They took him to another agent’s apartment and left the Chevy in the lot overnight.

Hours later, the case moved from a private struggle with addiction to an embarrassment for the FBI when, court documents say, agents found stolen drug evidence in Lowry’s car.

In the weeks that followed, criminal cases against 28 defendants, some of whom had pleaded guilty and been sent to prison, were dismissed after authorities deemed them tainted by Lowry’s alleged misconduct. Procedures at the Washington Field Office that allowed Lowry to repeatedly check drugs out of an evidence vault, and keep the packages for months without notice, were scrutinized, and changes were made.

A look at how Agent Lowry affected the case of Earl Owens, who was eventually dismissed.

Lowry was suspended. He has not been charged with a crime, although an investigation is continuing.

Lowry’s path to addiction, and the hours in which his alleged thefts were revealed, are detailed in a statement from his attorney and in more than 600 pages of documents obtained by The Washington Post, including internal FBI memos and transcripts of interviews with agents.

The documents tell the story of a struggling FBI agent who graduated with honors from the University of Maryland and was a team leader of his class at the FBI academy but who fell victim to some of the same addictions he had sworn to eradicate through law enforcement. His attorney says that, like many other heroin users his client encountered over his career, Lowry’s drug use began with a dependence on prescription painkillers.

It wasn’t until the day after the agents found Lowry in the lot at Seventh and L streets in Southeast that one of them discovered a bag of heroin under a seat of the dirty car.

Then the FBI agent found another bag. And another. And another.

“It’s way worse than we could have imagined,” that agent later told investigators.

The first time Lowry used heroin was the summer or fall of 2013 while on an investigation the FBI dubbed Midnight Hustle. He told investigators that he took a small amount of the drug obtained in an undercover buy.

Photographs from FBI evidence files of heroin found in the car assigned to FBI agent Matthew Lowry. Prosecutors dropped drug cases against 28 defendants in four cases based on evidence now deemed tainted. (U.S. District Court)

Lowry became addicted to pain medication in 2012, the documents state; the reason was not divulged. His attorney, Robert C. Bonsib, would say only that Lowry suffered from a “severe medical condition.” His primary doctor left private practice for a research facility, and a new doctor prescribed “power pain medications” that Bonsib said alleviated symptoms but failed to address the underlying issues.

He said that his client became addicted to the drugs and that when that doctor suddenly disappeared, Lowry tried to stop “cold turkey,” but “the addiction was overpowering and he began to self-medicate himself by removing small quantities of heroin from evidence seized during the course of certain narcotic investigations.”

An FBI memo says Lowry told investigators that he snorted the heroin at his home, up to about half a gram at a time. He used rolled-up paper as a straw.

After being discovered, Bonsib said, Lowry “acknowledged his addiction, sought treatment and agreed to fully disclose to prosecutors and the FBI investigators the specifics of his conduct. . . . He is devastated by the impact his conduct has had on the investigations in which he was involved and is committed to doing whatever he can to bring this investigation to a prompt and fair conclusion.”

Lowry’s wife and his father, speaking through Bonsib, declined interview requests. Citing the ongoing investigation, authorities at the FBI’s Washington Field Office also declined to comment for this article.

Lowry, who was assigned to a task force focused on crime along the District-Maryland border, managed to siphon drugs from evidence packages for about a year, forging the signatures of bosses and co-workers and taking advantage of rules that allowed a single agent to sign out drugs, according to FBI memos and court documents.

The last week of September was particularly stressful for Lowry, according to the FBI documents. He was late for several firearms training sessions, a prosecutor was pressing him to revise a wiretap warrant and he was worried that his boss had noticed a change in his behavior. He was frequently absent from the office, telling other agents that he hated desk work.

He, his wife of three years and their newborn were living at his father’s home in Upper Marlboro, Md., while their house was being built. Around that time, his wife, a pharmaceuticals company representative, went to a conference, and Lowry was responsible for child care. Lowry told the FBI that on Sept. 28, his wife walked out of his father’s house, texting him that he was not a good husband or father.

The next day, Lowry checked out heroin from the FBI evidence room. He drove out of downtown, through the Third Street Tunnel, and exited near the Navy Yard. He said he pulled over because he was tired; he also had run out of gas.

When he didn’t arrive home, Lowry’s father, William Lowry, called one of his son’s friends, a fellow agent. The agent said William Lowry described his son’s dependency on prescription drugs — with no mention of heroin — and the text his son received from his wife.

Lowry’s father told the agent, “You know, over the years, no matter what was going on in his life, you know, when I reached out to my son, he’d text me back.” This night, he received no response.

The agent, along with others who worried that Lowry might harm himself, had been trying, unsuccessfully, to call Lowry on his cellphone. Another colleague checked bars that Lowry frequented. About 8 p.m., the agent called again, and this time Lowry answered. He said his car had broken down and he was at a construction site.

“And I’m like, ‘We’re coming to get you,’ ” the agent said he replied.

The two stayed on the phone until the agent pulled into the lot. Other agents arrived, as did Lowry’s father. They took his guns and said that Lowry kept returning to the car, rummaging around inside. “He had a complete and total breakdown,” one agent said in a memo.

They decided to take Lowry to a colleague’s apartment in Annapolis.

The next morning, the agents went to retrieve the Chevy. They borrowed a gas can and put fuel in the empty tank.

Later, one of the agents began to throw out the trash from the car. That, the agent said, was when he found the first bag of heroin under the seat.

He called the agent who was with Lowry and told him about the drugs. That agent said he told Lowry: “Look, we know what was in the car. And it’s obviously a problem. And you’re going to go to rehab. . . . And I’m going to take you there, okay? Right now.”

The agent said Lowry answered, “Okay, that’s fine.”

Lowry’s father — who spent 27 years on the Prince George’s County police force before joining Anne Arundel’s — told the FBI that his son started a drug rehabilitation program Oct. 2, three days after the Navy Yard incident.

The agents who helped Lowry repeatedly told investigators that they were helping a friend through a personal crisis and never thought it involved stolen drugs. “We think we have an agent who, you know, had the wheels come off,” one said during his interview. “He’s drunk, he’s having marital problems, he’s got a 7-month-old at home. We want to help, you know, get him through it.”

Lowry’s attorney said his client wanted to follow in his father’s footsteps “since he was a little boy.” At the FBI academy, he had received the director’s leadership award. He completed a master’s program in finance. He led a covert surveillance group and a carjacking task force and worked in the counterintelligence squad. He mentored students in troubled D.C. neighborhoods.

But on Sept. 29, in the Navy Yard lot, Lowry stood with his worried co-workers and his father, his life falling apart.

An agent recalled, “I remember him giving a hug to his father.”

An earlier version of this article incorrectly stated Lowry’s FBI code name.