WASHINGTON (Reuters) - U.S. law enforcers have arrested dozens in a crackdown on "spam" e-mail, identity theft and other fraudulent online activity, a person involved in the investigation said Wednesday.

The U.S. Justice Department (news - web sites) was expected to announce details of the sweep at a press conference on Thursday. The department declined to comment before then.

Congress outlawed many forms of unsolicited bulk e-mail last year, but spam continues to swamp Internet users with unwanted pitches for pornography, questionable medicine and suspiciously low mortgage rates.

Unsolicited bulk e-mail accounted for 65 percent of all e-mail traffic, according to computer-security company Symantec Corp., up from 50 percent in July 2003.

Over the past year, fraudsters have used spam to spread computer viruses and craft fake notices from banks to trick people into giving up their credit card numbers, a tactic known as "phishing."

June saw 1,422 separate phishing attacks, up from 176 in January, according to the Anti-Phishing Working Group, a financial-services industry task force.

Internet providers like America Online have filed dozens of spam lawsuits over the past year, and the Federal Trade Commission for years has shut down marketers who send fraudulent or deceptive e-mail.

Criminal cases have been less common so far.

State prosecutors in Virginia, New York and a handful of other states have filed criminal suits against some spammers, and the Justice Department arrested four Detroit-area men for spamming activity in April.