The forecast called for record snowstorms, and Luis Octavio López Vega had no heat in his small hide-out.

Thieves had run off with the propane tanks on the camper that Mr. López had parked in the shadow of a towering grain elevator, near an abandoned industrial park. Rust had worn through the floor of his pickup truck, which he rarely dared to drive because he has neither a license nor insurance. His colitis was flaring so badly he could barely sit up straight, a consequence of the breakfast burrito and diet soda that had become part of his daily diet. He had not worked in months and was down to his last $250.

Going to a shelter might have opened him to questions about his identity that he did not want to answer, and reaching out to his family might have put them at odds with the law.

“I cannot go on like this, living day to day and going nowhere,” Mr. López, 64, said one night last winter. “I feel like I’m running in place. After so many years, it’s exhausting.”

Mr. López, a native of Mexico, said in Spanish that he has lived under the radar in the western United States for more than a decade, camouflaging himself among the waves of immigrants who came across the border around the same time. Like so many of his compatriots, he works an assortment of low-wage jobs available to people without a green card. But while Mr. López blends into that resilient population with his calloused hands and thrift-store wardrobe, his predicament goes far beyond his immigration status.

Mr. López played a leading role in what is widely considered the biggest drug-trafficking case in Mexican history. The episode — which inspired the 2000 movie “Traffic” — pitted the Mexican military against the United States Drug Enforcement Administration. Throughout the 1990s, Mr. López worked closely with them both. He served as a senior adviser to the powerful general who was appointed Mexico’s drug czar. And he was an informant for the D.E.A.

His two worlds collided spectacularly in 1997, when Mexico arrested the general, Jesús Gutiérrez Rebollo, on charges of collaborating with drug traffickers. As Washington tried to make sense of the charges, both governments went looking for Mr. López. Mexico considered him a suspect in the case; the D.E.A. saw him as a potential gold mine of information.

The United States found him first. The D.E.A. secretly helped Mr. López and his family escape across the border in exchange for his cooperation with its investigation.

Dozens of hours of testimony from Mr. López about links between the military and drug cartels proved to be explosive, setting off a dizzying chain reaction in which Mexico asked the United States for help capturing Mr. López, Washington denied any knowledge of his whereabouts and the D.E.A. abruptly severed its ties with him.

The reserved, unpretentious husband and father of three has been a fugitive ever since, on the run from his native country and abandoned by his adopted home. For more than a decade, he has carried information about the inner workings of the drug war that both governments carefully kept secret.

The United States continues to feign ignorance about his whereabouts when pressed by Mexican officials, who still ask for assistance to find him, a federal law enforcement official said.

The cover-up was initially led by the D.E.A., whose agents did not believe the Mexican authorities had a legitimate case against their informant. Other law enforcement agencies later went along, out of fear that the D.E.A.’s relationship with Mr. López might disrupt cooperation between the two countries on more pressing matters.

“We couldn’t tell Mexico that we were protecting the guy, because that would have affected their cooperation with us on all kinds of other programs,” said a former senior D.E.A. official who was involved in the case but was not authorized to speak publicly about a confidential informant. “So we cut him loose, and hoped he’d find a way to make it on his own.”

These are the opaque dynamics that undermine the alliance between the United States and Mexico in the war on drugs, a fight that often feels more like shadow boxing. Though the governments are bound together by geography, neither believes the other can be fully trusted. Mr. López’s ordeal — pieced together from classified D.E.A. intelligence reports and interviews with him, his family, friends, and more than a dozen current and former federal law enforcement officials — demonstrates why the mutual distrust is justified.

The absence of any facts to either condemn Mr. López or exonerate him of corruption has wrought havoc on the former informant, and his fugitive’s existence has been a ball and chain on his family, whom he sees during sporadic rendezvous. They all exhibit symptoms of emotional trauma, bouncing among flashes of rage, long periods of depression, episodes of binge drinking and persistent paranoia.

During several long interviews, Mr. López repeatedly said he was not guilty of any wrongdoing. He said he has refused to turn himself in to the Mexican authorities because he believes he will be killed rather than given a fair hearing. But years of living an anonymous, circumscribed life have been nearly as suffocating as a jail cell.

He starts most mornings at McDonald’s, where breakfast costs less than $2 for seniors and free Wi-Fi allows him to peruse Mexican newspapers on his battered laptop for hours, his mind replaying the life choices that landed him there.

“I risked my life in Mexico because I believed things could change. I was wrong. Nothing has changed,” Mr. López said. “I helped the United States because I believed that if all else failed, this government would support me. But I was wrong again. And now, I’ve lost everything.”

The Military Steps In

These days, Mr. López wonders whether he is losing his mind as well. Last September, he took his troubles to a psychiatrist at a health clinic, telling her how his emotions were running erratically from hot to cold and about his difficulty sleeping. An hour later, he left with a diagnosis of bipolar disorder and a bottle of pills he decided not to take.

Sipping Diet Coke in a sunlit hotel room, Mr. López explained that he felt it was riskier to become dependent on medication that could be confiscated if he fell into police custody. More important, he said, the whole diagnosis was based on a lie — one of the many he tells to get by each day. When the doctor asked him what might be causing his stress, he told her that his family had turned against him.

“Imagine telling her what is really going on in my life,” Mr. López said. “Where would I start? That I once helped capture El Güero Palma, and now I’m being treated like a delinquent?”

Ballads were written in Mexico about the day in 1995 when the authorities took down Héctor Luis Palma Salazar, known as “El Güero,” the fearsome kingpin of the Sinaloa cartel. Mr. Palma met his fate on the outskirts of Guadalajara in suburban Zapopan, a nexus for everybody who was anybody in the drug war.

Mr. López served nearly two decades in the municipal police department there, most of them as chief. Politically astute and streetwise, he caught the attention of the D.E.A., which developed him as a confidential source during the mid-1990s and valued him for the reliability of his information.

Drug violence was raging. When things got too heated, Mr. López sought backup from General Gutiérrez, a powerful ally whose territory spanned five Mexican states. It was part of a secret arrangement, Mr. López said, in which his officers shared information about the cartels with the military and the general provided extra muscle to the Zapopan police.

At home, Mr. López’s wife and three children lived surrounded by bodyguards and snipers. With her husband often absent, Soledad López had her hands full with the children. Their oldest child, David, got his high school girlfriend pregnant. Luis Octavio failed eighth grade three times. Cecilia, the youngest, did not understand the tumult around her, and Mrs. López worked to protect her from it.