The Federal Bureau of Investigation has opened an inquiry into whether several eBay users, including the Sacramento lawyer who almost sold a colorful abstract painting for $135,805 after putting it up for sale for 25 cents, committed fraud by bidding up the prices of one another's online auction offerings.

Donald Vilfer, a supervisory special agent in the F.B.I.'s Sacramento office, said the bureau turned its attention to the case after reading a June 2 article in The New York Times. That article outlined how Kenneth A. Walton, the Sacramento lawyer, and several other eBay users had engaged in cross-bidding on one another's items and offered glowing testimonials to each other on the eBay site. Mr. Walton has denied operating in concert with others.

The bureau's office in Sacramento has been watching the episode since last month, when eBay voided the sale and suspended Mr. Walton for bidding on his own painting, whose price had skyrocketed on speculation that it might be by the renowned California artist Richard Diebenkorn. Self-bidding, known as shill bidding, is forbidden by eBay rules and is generally illegal in the traditional auction world.

Participation in a shill-bidding ring would run afoul of federal statutes prohibiting mail fraud and wire fraud; each count could carry a maximum penalty of up to five years in prison and $1 million in fines.