SAN DIEGO — For years, Benjamin Arellano Felix eluded U.S. law enforcement while running a Mexican drug cartel that terrorized rivals and poured hundreds of tons of cocaine into the country. So when the handcuffed kingpin arrived in San Diego aboard a government plane last year, U.S. authorities gathered on the tarmac, sharing hugs and handshakes as he was handed over to his longtime pursuers.

But the sense of triumph has turned to disappointment in some quarters as Arellano Felix approaches his judgment hour in court Monday. A judge is expected to sentence him to a prison term of 25 years maximum, a fraction of what he could have received for his role in heading one of Mexico’s most powerful and ruthless organized crime groups.

Arellano Felix, the 60-year-old former head of the Tijuana-based cartel that bears his family name, may be in his early 80s when he is released. He would be sent to Mexico, where he faces more prison time, but that’s cold comfort to critics, among them family members of victims and law enforcement officers who dedicated their lives to wiping out his criminal empire.

Legal observers speculate that prosecutors negotiated the plea agreement to avoid a potentially long and costly trial, or because evidence from crimes that stretch back to 1991 had gone stale.

Laura Duffy, the U.S. attorney in San Diego who gained prominence for her tough leadership of the sprawling investigation, declined to comment until after the sentencing, as did other prosecutors.

Critics contend the investigation was airtight. The sentence, they say, undercuts a case of historic significance that targeted a man whose ruthless leadership set the standard for the kind of savagery that grips areas of Mexico.

“The man is a monster; there’s no way around it,” said John Ziegler, a retired Drug Enforcement Administration special agent who worked on the case for several years. For a trafficker of Arellano Felix’s stature, he said, a 25-year sentence undermines U.S. counter-narcotics efforts. “If you’re a crook doing terrible things and see that Benjamin gets 25 years — that doesn’t exactly inspire fear into you.”

The DEA-led investigation of the Tijuana cartel was among the biggest ever undertaken against Mexican organized crime, spanning two decades. It involved a multitude of local and federal agencies and featured unprecedented cooperation from Mexican law enforcement officials, some of whom were brutally slain by cartel assassins who also turned their guns on the news media and religious leaders.

Arellano Felix was captured by Mexican authorities in 2002 and extradited to the United States last year. In January, he pleaded guilty to racketeering and money-laundering charges, admitting that the organization he led generated hundreds of millions of dollars in profits that it used to bribe Mexican officials and equip assassination squads with weapons that easily overmatched police firepower.

The cartel was responsible for as many as 1,000 homicides, according to investigators, and it popularized the grim practice of dissolving victims in barrels of caustic liquids. Many of the cartel’s enforcers were San Diego gang members, some trained by Middle Eastern mercenaries.

Weighing whether to go to trial against a high-profile organized crime figure can be a tricky calculation. Prosecutors have to consider factors including the safety of witnesses and their families and the potential of gaining valuable intelligence in exchange for a reduced sentence.

“Sometimes you have to look at the bigger picture,” said Mark Adams, a San Diego criminal defense attorney. He said the logistics of bringing in witnesses from prisons around the country would have posed a security challenge at the downtown courthouse and neighboring federal correctional center.

Some question whether the evidence would have been strong enough, given that many witnesses would have been trading cooperation for reduced sentences. Those who may have testified represented a who’s who of the cartel’s leadership ranks, including Arellano Felix’s brother Javier, who is serving a life term.

Law enforcement officials familiar with the case noted, however, that prosecutors regularly gain convictions based on testimony of criminals. The sheer number of witnesses would have eliminated any doubt of guilt, they said.

“Every law enforcement person I talk to finds this absolutely baffling,” said Nathan Jones, an adjunct lecturer at the University of San Diego who wrote his dissertation on the cartel.

There’s no shortage of speculation about why prosecutors didn’t go to trial: Did they want to avoid revealing evidence that would have implicated political, law enforcement and church figures in Mexico? Was there concern that defense attorneys would embarrass them by showing “trophy” snapshots authorities took with their arms around Arellano Felix after his extradition?

Several drug kingpins of Arellano Felix’s stature have received stiffer sentences. A former head of the Gulf cartel, Juan Garcia Abrego, received 11 consecutive life terms. Colombian drug lord Carlos Lehder got a life sentence, which was reduced to 55 years after he testified against former Panamanian leader Manuel Noriega.

But Osiel Cardenas Guillen, another former Gulf cartel boss, received a 25-year term. The extraordinary secrecy surrounding his case — the sentencing hearing in 2010 was closed to the public — suggested that he cooperated with authorities.

Anthony Colombo, the San Diego defense attorney who negotiated Arellano Felix’s plea deal, insisted there was no cooperation agreement. Colombo said that a trial could have lasted as long as six months and that prosecutors would have had to prove about 30 murders.

In the end, he said, prosecutors may have balked at the expense, especially if they believed that Arellano Felix’s days in jail would be his last. “I mean, he’s almost going to be 80 years old,” Colombo said. “It’s likely in their view that he may not survive prison.”

richard.marosi@latimes.com