Dressed in business attire, he would casually stroll into office buildings throughout Northern Virginia. Then, police said, the "office larcenist" would strike.

He would enter an office and grab a wallet from someone's purse or suit jacket. Within minutes, the victim's cash would be spent and their credit cards used.

Fairfax County police yesterday charged Ricardo A. Jones, 38, of Southeast Washington with fraud, forgery and grand larceny, all felonies. He is suspected in at least 50 -- and perhaps more than 100 -- thefts of wallets from offices, mainly in Fairfax but also in Arlington and Prince William counties.

Fairfax police said that the larcenies began last year and that the most recent Fairfax theft was Oct. 13. The targets have included the offices of doctors and real estate agents. The only apparent pattern has been that the larcenist has focused on tall buildings with no visible security, Fairfax police spokeswoman Mary Mulrenan said. He has tended to target women's purses, she said, and sometimes has hit more than one office in the same building.

"He would dress conservatively, not in a suit but in a way where he wouldn't stand out in an office environment," Mulrenan said. "He just blended in. People think five minutes isn't a lot of time to be away from your wallet, but this guy needed just 30 seconds."

It was unclear, police said, precisely how the larcenist gained access to so many offices at a time of heightened security since the Sept. 11, 2001, terrorist attacks. In general, Mulrenan said, he has focused on offices that seemed vulnerable. "If there was an office where you needed an ID to get in, he wouldn't do it," she said.

While police encouraged offices and their workers to tighten security, business groups said they already have been doing that. But the emphasis has been on preventing large-scale incidents, such as a terrorist attack or cyber-attacks on office computer networks.

"Our members are very concerned about security, but sometimes they are focused on the big threats and maybe not as focused on . . . I hate to call it the innocuous threats," said Tony Howard, spokesman for the Fairfax County Chamber of Commerce.

The chamber recently sponsored a discussion about business security attended by about 200 people. The speaker: the State Department's chief of counterterrorism.

Jones was arrested last Thursday in the Rosslyn section of Arlington County after Fairfax police contacted Arlington detectives, Arlington police spokesman Matt Martin said. He was arrested while getting into his car with two wallets that didn't belong to him, Martin said.

"He was caught red-handed," said Martin, who added that Jones is suspected of committing more than a dozen office larcenies in Arlington. Jones, who lives in the 4600 block of Martin Luther King Jr. Avenue SE, is being held without bond at the Fairfax jail. Police declined to say how they came to suspect Jones.

A Prince William police spokesman did not return telephone calls.

In Fairfax, police said the larcenist worked mainly in the Fair Oaks and McLean areas. The larcenist would strike throughout the workday, at no particular time, and would move quickly. One woman in Oakton left her office from 10:30 to 10:45 a.m. and noticed her wallet was missing an hour later.

Her credit cards already had been used.

Mulrenan said the larcenist was not particularly extravagant in his spending of other people's money. Usually, he would buy such things as groceries or gift cards or go to the hardware store. "It's almost like this was his livelihood," Mulrenan said.

Police encouraged businesses to tighten security and said workers should keep better tabs on their wallets and more aggressively question strangers. "People need to be more vigilant in offices," Martin said.