While it's possible San Francisco's Barry Bonds could be charged with perjury, lawyers and former federal prosecutors say it would be difficult to get a conviction.

SAN FRANCISCO — Even if Giants slugger Barry Bonds is charged with lying to a grand jury, it will be hard to convict him, former federal prosecutors and other lawyers said.

"It is a lot tougher to make a perjury case than most people think because it takes more than just proving that the person made a statement that was untrue," said Adam Hoffinger, a criminal defense lawyer in Washington D.C. "The government has to prove that he knowingly and willfully lied about a material fact — it can't be a mistake, there has to be intent."

A federal grand jury is investigating whether Bonds committed perjury when he testified in 2003 that he never used steroids, a person with knowledge of the probe told The Associated Press on Thursday night. The person spoke on condition of anonymity because of the secrecy of the investigation.

Bonds, who is chasing Hank Aaron's home run record, was granted immunity to testify truthfully before a grand jury in December 2003 investigating a Northern California steroid distribution ring based at the company called the Bay Area Laboratory Cooperative, or BALCO.

According to excerpts of testimony previously reported by the San Francisco Chronicle, Bonds testified that he used a clear substance and a cream given to him by a trainer convicted in the case, but said he didn't know they were steroids.

Bonds testified that Greg Anderson, his personal trainer, told him the substances were the nutritional supplement flaxseed oil and an arthritis balm.

Anderson and three others, including BALCO founder Victor Conte, have pleaded guilty to distribution charges.

Erwin Cherminsky, a Duke University criminal law professor, said the excerpt he has read of the recently published book "Game of Shadows," which details extensive and knowing steroid use by Bonds, "certainly suggests he lied under oath."

The book's publication prompted Major League Baseball Commissioner Bud Selig to launch an investigation into steroid use among big league players.

Michael Rains, Bonds' attorney, did not return a telephone call Friday.

Experts said prosecutors will need some evidence other than Bond's testimony that he didn't know what he was taking to prove he purposely lied to mislead the grand jury.

The fact that the U.S. attorney's office in San Francisco has convened a grand jury to look into Bond's steroids testimony means the slugger is probably in trouble, he said.

"They wouldn't go in front of the grand jury unless they had something to go after," Horowitz said.

A spokesman for the U.S. attorney's office didn't return a telephone call seeking comment.