By JAMES C. McKINLEY Jr.
 
MEXICO CITY, Jan. 31 - After eight years in flight, a man prosecutors in the United States say was the main financial mastermind for the infamous Ciudad Juárez drug cartel of Amado Carrillo Fuentes has been arrested in Mexico and is awaiting extradition, prosecutors here said Monday.

The suspect, Juan José Álvarez Tostado, was the main target of a huge sting operation by United States customs officials in the 1990's. The Mexican police said they apprehended him on Thursday as he drove through the capital in his car.

"He's been on our 10-most-wanted-fugitives list ever since he was indicted in May 1988," said Dean Boyd, a spokesman for the Bureau of Immigration and Customs Enforcement in Washington. Mr. Boyd said United States officials were still trying to confirm that the man in custody was Mr. Álvarez Tostado, though Mexican officials said they had confirmed it.

The arrest came a day before another fugitive, Augustín Vásquez-Mendoza, wanted in the United States on charges of ordering the killing of a drug enforcement agent, Richard Fass, in Arizona 10 years ago, was extradited from Mexico after a long legal battle, Drug Enforcement Administration officials said.

The 1990's sting operation resulted in more than 167 arrests and smashed an extensive money laundering system that involved three of Mexico's top banks. But it also caused a diplomatic uproar, as Mexican officials complained of being kept in the dark about the role of American undercover agents, who the Mexicans contended had run roughshod over Mexican law.

Several top bankers were arrested after customs agents flew 44 people suspected of involvement in the money-laundering scheme on a private plane from Mexico to a brothel near Mesquite, Ariz. Most were later convicted.

But Mr. Álvarez Tostado, the central figure of the case, remained underground for years, wanted in the United States and Mexico on money laundering charges. American investigators say he was responsible for collecting money for the Juárez cartel from drug sales in the United States and transferring it to Mexico and Colombia.