SAN DIEGO — A federal grand jury in San Diego is taking testimony on allegations that eight present and former U.S. Customs Service employees, including two criminal investigators, have been assisting Mexican drug cartels in shipping tons of cocaine into the United States, officials said.

Informants have told investigators that Customs officials in San Diego were paid off with bribes and sexual favors, sources said. Authorities are performing an extensive examination of the personal finances of those officials, sources said.

The investigation, directed by the FBI in San Diego and the Justice Department in Washington, was launched after The Times reported in February that there were no cocaine seizures in 1992 and 1994 at the three largest commercial ports on the Southwest border.

The nexus of the probe is Customs' controversial line-release program, which allows thousands of Mexican trucks to enter the country without inspections each month, sources said. The investigation stems in part from allegations of former Customs inspector Mike Horner of San Diego, U.S. Atty. Alan D. Bersin said.

It represents an escalation of federal examinations of alleged corruption within the Customs Service. Last month, two inspectors in El Paso were charged with conspiring to smuggle 2,200 pounds of cocaine into the country in return for a $1-million payoff from drug smugglers.

In a separate action, the Justice Department announced last week that three former federal prosecutors in Miami were indicted for helping Colombian cocaine dealers avoid prosecution in the United States.

Last month, the Justice and Treasury departments reached an unprecedented agreement, giving the FBI sole authority to investigate corruption cases in the Customs districts in San Diego and Los Angeles. On Thursday, Justice Department spokesman John Russell in Washington declined to comment on the grand jury investigation in San Diego.

Customs spokeswoman Bobbie Cassidy in San Diego also declined to comment, but said Customs officials welcomed the investigation. She declined a Times request for interviews with several employees identified as targets of the probe.

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Sources said the FBI is looking into allegations that Customs officials with ties to San Diego and supervisory inspectors in key enforcement positions aided and abetted drug smuggling through four ports of entry on the California-Mexico border, law enforcement sources said.