The Associated Press
FILE - In this March 9, 2002 file photo released by Mexico's Attorney General's Office, Benjamin Arellano Felix stands in his home the day of his arrest in Puebla, Mexico. Arellano Felix will plead guilty to unspecified charges, the U.S. attorney's office in San Diego said Wednesday, Jan. 4, 2012. Spokeswoman Debra Hartman said she could not elaborate in advance of the filing. Arellano Felix is expected in federal court Wednesday afternoon. Arellano Felix headed a once-mighty cartel that came to power in Tijuana, Mexico, in the late 1980s. (AP Photo)

SAN DIEGO — Mexican drug kingpin Benjamin Arellano Felix stood attentively in court, acknowledging his guilt as a federal judge read a plea agreement that detailed his role at the helm of one of the world's most powerful cartels in the 1990s.

He told the judge he has been suffering migraines almost daily but that his headaches didn't impair his judgment to accept the prosecutors' offer.

Wednesday's half-hour hearing was an anticlimactic finish to the U.S. government's pursuit of the head of an organization that smuggled hundreds of tons of cocaine and marijuana to the United States. His cartel, which once had an iron-tight grip on the drug trade along California's border with Mexico, has struggled in recent years as other cartels have become more ruthless than ever.

Under an agreement with federal prosecutors, Arellano Felix, 58, can be sentenced to no more than 25 years in prison — a lighter punishment than ordered for lower-ranking members of his once-mighty, Tijuana-based cartel.

Prosecutors agreed to dismiss other charges that could have brought 140 years in prison if he was convicted.

Lawyers who have followed the case said the lighter sentence may be because the alleged crimes occurred many years ago and relied on witness accounts, instead of wiretaps or physical evidence.

John Kirby, a former federal prosecutor who co-wrote the 2003 indictment against Arellano Felix, said those cases weaken over time as witnesses die, get into more trouble or change their minds about testifying.

"This kind of case is based solely on witness testimony, and it slowly disintegrates," he said. "Maybe from the time when we put it together and now, it's not such a great case anymore."

Laura Duffy, the U.S. attorney in San Diego who built much of her career on the case, said Arellano Felix will likely spend the rest of his life in U.S. prison.

"Today's guilty plea marks the end of his reign of murder, mayhem and corruption, and his historic admission of guilt sends a clear message to the Mexican cartel leaders operating today: The United States will spare no effort to investigate, extradite and prosecute you for your criminal activities," Duffy said.

Anthony Colombo Jr., Arellano Felix's attorney, said his client could be released from U.S. prison in 20 years if credited for time served in this country and good behavior, assuming he gets the maximum 25-year sentence. As a Mexican citizen, he would then be deported to Mexico, where he still has nine years left on a sentence for related crimes.

Colombo said the government may have agreed to the deal to avoid having to bargain with 21 potential government witnesses for reduced sentences in exchange for their testimony.

Francisco Javier Arellano Felix, a younger brother who led the cartel after Benjamin was arrested in Mexico in 2002, was sentenced in San Diego to life in prison in 2007, a year after he was captured by U.S. authorities in international waters off Mexico's Baja California coast. Jesus Labra Aviles, a lieutenant under Benjamin Arellano Felix, was sentenced in San Diego to 40 years in prison in 2010.

Benjamin Arellano Felix was extradited from Mexico in April 2011 to face drug, money-laundering and racketeering charges, one of the highest-profile kingpins to face prosecution in the United States.

The U.S. indictment said Arellano Felix was the top leader of a cartel he led with his brothers, going back to 1986. It says the cartel tortured and killed rivals in the United States and Mexico as it smuggled Mexican marijuana and Colombian cocaine.

The cartel, portrayed in the Steven Soderbergh film "Traffic," lost its grip after Benjamin Arellano Felix was arrested in 2002. A month earlier, his brother, Ramon, the cartel's top enforcer, died in a shootout with Mexican authorities.