The recent unauthorized access of about 8 million credit-card accounts nationwide is one of the largest breaches of credit-card security in memory.

But the intrusion, disclosed by credit-card firms Tuesday, is just one of many "cybercrimes" that have increased with the growth of the Internet, according to authorities.

A local computer-security expert said it remains difficult to make computer systems hacker-proof, although it helps when early-warning programs are put in place.

"There's so many vulnerabilities in technology today, the opportunities to attack are so prevalent," said Reed Harrison, chief technology officer and a founder of e-Security Inc., a maker of computer-security software in Rockledge.

"It's very tough for a large company, with the sheer magnitude of information involved, to protect all of their information all the time," Harrison said.

An "unauthorized intruder" accessed at least 8 million credit-card account numbers -- including those of American Express, MasterCard and Visa, the companies said Tuesday. They said the intruder breached the security of a company that processes credit-card transactions for merchants.

The breach included an estimated 3.4 million Visa cards and 2.2 million MasterCards. As of Tuesday, the companies said they were not aware of any theft or fraud from the accounts.

American Express said security processes were in place to determine if card numbers were being misused, but that "we're not aware of any unusual activity with the affected cards."

Visa said it "has been informed by a third-party payment processor about an unauthorized intrusion." It said that, after learning of the incident, the company's fraud team "immediately notified all affected card-issuing financial institutions and is working with the third-party-payment card processor to protect against the threat of a future intrusion."

MasterCard said investigations were under way.

"We have notified our member financial institutions of the accounts involved, so that they may monitor each account for fraud and/or reissue cards as appropriate," the company said.

Some cases of credit-card information hackers have been costly.

In March 2000, for example, a computer hacker allegedly broke into e-commerce Web site in the United States, Canada, Japan, Thailand and the United Kingdom. The hacker apparently stole up to 28,000 credit-card numbers, with resulting losses estimated to be at least $3.5 million, according to the Federal Bureau of Investigation.

In May 2000, federal authorities opened the Internet Fraud Complaint Center. In its first year, the center received 30,503 valid criminal complaints, said Thomas Kubic, deputy assistant director of the FBI's Criminal Investigative Division.

"The use of the Internet for criminal purposes is one of the most-critical challenges facing the FBI and law enforcement in general," Kubic told members of Congress in 2001.

Since then, the U.S. Department of Justice has recorded an array of computer crimes.

In November, for example, federal authorities in New York charged a man with wire fraud and conspiracy for using a computer to access consumer-credit reports of more than 30,000 people, then selling the reports on the street for his own profit.

Authorities believed it was the nation's largest-ever case of identity theft.