SAN FRANCISCO — The federal investigation of Barry Bonds approaches a key juncture today, with the grand jury probing the baseball star for perjury and tax evasion set to expire and a possible indictment of Bonds looming.

Bonds’s personal trainer, Greg Anderson, also was awaiting a key ruling from a federal appeals court that could release him from prison, where he was sent earlier this month after refusing to testify against his childhood friend.

Anderson likely holds the key to whether perjury charges could stick against Bonds, who testified in 2003 that he thought substances given to him by the trainer were arthritis balm and flaxseed oil.

Authorities suspected Bonds was lying and that those items were “the clear” and “the cream” — two performance-enhancing drugs tied to the BALCO, the lab exposed as a steroids supplier to top athletes.

“Obviously, they think they need Greg to prove perjury,” Mark Geragos, Anderson’s lawyer, said yesterday.

Allegations of steroid use long have plagued Bonds, who passed Babe Ruth in May to become second only to Hank Aaron on the career home run list.They intensified in late 2003, when he testified before the original Bay Area Laboratory Co-Operative grand jury.

Anderson’s status is one of many legal uncertainties surrounding the San Francisco Giants slugger.

Without the trainer’s help, prosecutors still could indict Bonds on charges alleging he failed to pay taxes on money made through sales of autographs and other memorabilia. They also could seek to extend the grand jury’s term to put more pressure on Anderson to cooperate, or convene a new panel and put Anderson back in jail. There’s also the chance Bonds might be indicted on perjury charges without Anderson’s testimony.

Anderson was one of five men convicted in the steroids scandal surrounding BALCO. He was sentenced to three months behind bars and three months of home confinement in October after pleading guilty to money laundering and steroid distribution. He was found in contempt of court and jailed again July 5 for refusing to testify in the Bonds probe.

Federal prosecutors say they need Anderson, in part, to interpret calendars that seem to spell out Bonds’s schedule for using performance-enhancing drugs. The calendars were seized by investigators from Anderson’s house in 2003.

Geragos says Anderson must be released when the grand jury’s term expires today, even if prosecutors succeed in extending the panel’s investigation.

Separately, Geragos has launched an effort to get his client freed based on a tape-recorded conversation that Geragos says was made illegally in the spring of 2003 by government investigators. On the tape, Anderson allegedly discusses Bonds’s illegal drug use with an unidentified athlete.

“Mr. Anderson allegedly makes numerous remarks regarding baseball’s steroids testing, Barry Bonds’s use of an undetectable performance-enhancing drug to beat drug tests, and Mr. Anderson’s own alleged steroid use” Geragos said in a court filing.

Geragos is demanding that the government disclose the contents of that tape. He suspects they won’t and says it’s illegal for Anderson to remain in prison because he won’t testify about information the government allegedly obtained without a warrant.

“They have to turn over the tape or let Greg out,” Geragos said.

Former federal prosecutors said Anderson faces long odds with that argument because grand jury witnesses aren’t entitled to see the government’s evidence before they testify, except on rare occasions.

Geragos argued this is such an occasion, because he says the tape Anderson’s acquaintance made is an illegal wiretap. If the appeals court agrees, then Anderson would not have to testify, according to Geragos.

That scenario could jeopardize the government’s perjury investigation, he added.