By John Shiffman and Gabriel Stargardter
WASHINGTON/MEXICO CITY (Reuters) - The seizure of a phone belonging to the son of Joaquin "Shorty" Guzman's deputy at the U.S.-Mexico border was an important break in the operation that led to the drug lord's capture, a senior U.S. law enforcement official said on Sunday.
Guzman, who long ran the feared Sinaloa Cartel and was Mexico's most wanted criminal, was caught on Saturday in his native northwestern state of Sinaloa with help from U.S. agents.
It was a major victory for the Mexican government in its fight against powerful drug gangs and for the cause of cooperation between Mexican and U.S. security forces.
The phone that helped lead to Guzman's downfall belonged to the son of his deputy, Ismael "El Mayo" Zambada, who could now be in line to take over from his boss.
The break came when Zambada's son, Serafin Zambada-Ortiz, was arrested in November trying to cross the border from Mexico into the United States, where he faced sealed drug charges.
"This was one of several important turning points. But it was critical," the official said.
A lawyer for Zambada-Ortiz, Michael McDonnell of La Habra, California, said no data from his client's phone or other electronics led U.S. authorities to Guzman.
"He didn't know him ... His father did," McDonnell said. "I don't know where you're getting your information but Serafin Zambada had no connection to Guzman's arrest, period."
U.S. prosecutors said on Sunday they plan to seek the extradition of Guzman to face trial in the United States.
Robert Nardoza, a spokesman for the U.S. Attorney's office in Brooklyn, New York, said his office would request Guzman's extradition to face a variety of charges.
A spokesman for the Mexican attorney general's office declined to comment on the extradition request. President Enrique Pena Nieto's office also declined to comment.
Sensitivities over the issue could mean Guzman is more likely to face justice first in Mexico, where he still has an outstanding term to finish. He broke out of prison, reportedly in a laundry cart, in 2001.
The United States had a $5 million bounty on Guzman's head. His cartel has smuggled billions of dollars' worth of cocaine, marijuana and methamphetamine into the United States, and fought vicious turf wars with other gangs across Mexico.
Zambada-Ortiz, who is a U.S. citizen, entered the United States at Nogales, Arizona, to take care of a visa matter for his wife, the U.S. official said.
He apparently did not know that he was facing sealed cocaine and methamphetamine charges in San Diego when he crossed the border.
Immigration and Customs Enforcement agents arrested him and seized his phone. The Drug Enforcement Administration then compared the numbers in the phone with a database of more than 1 billion records, which includes information collected by subpoena, search warrants and arrests during other drug investigations.
The DEA also receives information on Mexican cartel telephone and email data from the U.S. National Security Agency, but U.S. officials declined to say whether the NSA played a role in the case.
"We handled this case like we handled many: using technology to work up the chain, person by person, to the top," the U.S. official said.
Last week, that trail led them to some of Guzman's senior henchmen but the drug boss himself narrowly escaped, using a network of tunnels and sewers to give his pursuers the slip.
Guzman, 56, was eventually captured on Saturday in a pre-dawn raid on a seaside condominium in the northwestern tourist resort and fishing and shrimp-processing center of Mazatlan, 135 miles from Guzman's suspected base in Culiacan.
"This is the biggest success in the drug war in 20 years, and shows that contrary to what you hear in the press, behind the scenes the U.S. and Mexico have been working well together," said the U.S. official.
In addition to facing U.S. criminal charges in Chicago and New York, Guzman was indicted in 2007 in Miami for cocaine smuggling with additional charges added last month.
Guzman also was charged in 2012 in Texas with importing cocaine and marijuana, money laundering, firearms violations and running a criminal enterprise that included murder.
More than 80,000 people have been killed in Mexico's drugs war over the last seven years with much of the violence in western and northern regions that have long been smuggling routes.
Many of the victims are tortured and beheaded and their bodies dumped in public places or in mass graves. The violence has ravaged border cities and even beach resorts such as Acapulco.
(Additional reporting by Simon Gardner in Mexico City and Mark Hosenball in Washington; Editing by Kieran Murray, Grant McCool and Mohammad Zargham)