A United Airlines baggage handler at Dulles International Airport pleaded guilty yesterday to stealing mail containing credit cards that passed through the airport, a scheme that authorities said defrauded financial institutions of nearly $4 million.

Emmanuel Osho, 49, admitted that he stuffed huge quantities of mail into a black duffel bag and drove the letters to a co-conspirator in New York. The credit cards inside were then activated and distributed to teams of people, called "strikers,'' who used them to get fraudulent cash advances at banks and credit unions in about 100 towns along the East Coast.

The stolen credit cards were intended for about 2,000 people, many in Northern Virginia. Although their Social Security numbers were obtained and used to activate the cards, all of the financial loss was absorbed by seven credit card companies and 56 credit unions, prosecutors said.

The case is the latest example of a federal crackdown on airport security triggered by the Sept. 11, 2001, terrorist hijackings. More than 140 employees at the Washington area's three major airports were charged in 2002, for example, with lying about their identities or criminal pasts on applications to work near airplanes, runways and cargo.

"We have to pursue cases like this because we have to ensure that the airports are safe and secure,'' Paul J. McNulty, the U.S. attorney in Alexandria, said yesterday as he announced Osho's plea at a news conference. The investigation into the credit card scam began in 2000, but the pace quickened after Sept. 11, 2001, said law enforcement officials, who spoke on condition of anonymity because the information was not in official court documents.

Court documents make no mention of terrorism in Osho's case, and officials said the scam was limited to stealing credit cards. Assistant U.S. Attorney Andrew E. Lelling said in court that authorities believe that at least one other Dulles employee was involved and that the investigation is continuing.

Osho, a Nigerian citizen, pleaded guilty to one count of wire fraud in U.S. District Court in Alexandria. Fighting back tears, he made a brief statement to Judge T. S. Ellis III that was mostly inaudible. He faces up to 30 years in prison when sentenced Oct. 14.

Bruce Johnson, Osho's lawyer, said afterward that his client had exercised "poor decision making" and is "deeply sorry for his involvement.'' But he said he plans to challenge the overall loss from the scam at Osho's sentencing, saying it was actually less than $1 million.

"He only wants to be held accountable for what he did,'' Johnson said. Prosecutors said Osho pocketed about $50,000.

Osho's co-conspirator in New York, Ademola Idowu, pleaded guilty in federal court in Alexandria for his role and was sentenced last month to 63 months in prison. Several of the dozens of strikers who actually used the cards have been caught and prosecuted on local charges in various cities, prosecutors said.

Jeff Green, a spokesman for United, said that Osho has been fired and that the company believes the scam was "an isolated incident.'' Everyone who works at an airport undergoes a strict background check, he said.

Large amounts of mail are routed through Dulles on commercial carriers such as United, bound for Northern Virginia or elsewhere on the East Coast. As pallets of mail sat on the tarmac awaiting transfer to mail facilities or connecting flights, court documents said, Osho would steal as many envelopes as he could.

Asked in court how he could tell which letters contained credit cards, Osho said only: "by searching.''

McNulty said the case "illustrates a very important point about some vulnerabilities in our system.''

But officials of the U.S. Postal Inspection Service, which investigated the scam after a tip from at least one credit card company, called it relatively isolated. They said credit card losses from mail that passes through Dulles have declined dramatically since Osho was arrested and charged in April.