SAN FRANCISCO -- BART officials warned riders Thursday that activists may stage surprise demonstrations during the coming days in a bid to disrupt service during commute times, but the evening rush hour came and went with no such protests.

The agency did not say why it was concerned, but a recent posting on one protest website - since removed by the site - asked people to meet Thursday afternoon at Civic Center Station in San Francisco, where a BART police officer fatally shot a knife-wielding man, Charles Blair Hill, on July 3.

Activists also remain upset about the fatal shooting of unarmed BART passenger Oscar Grant on New Year's Day 2009 by a former officer who was convicted of involuntary manslaughter.

"We wish to remain inconspicuous until the action begins at 5 p.m.," the posting by the protest group said. It added that activists should "try to mobilize without public announcement beforehand. This will allow us the element of surprise, and BART will not be able to call in their police force to harass our event."

BART warned passengers and beefed up its police presence in an effort to take away that element of surprise. By 5 p.m., there were plenty of police officers with riot helmets at the ready and members of the media at Civic Center Station, but no protesters.

"We do anticipate a group of people are going to try to disrupt service, and we're going to try to do our best to prevent that," BART's deputy police chief, Ben Fairow, said earlier.

Protesters briefly shut down the Civic Center, Powell Street and 16th Street Mission stations July 11. Trains ran through the stations without stopping.

The July 3 shooting remains under investigation. According to BART, Officer James Crowell shot Hill, a 45-year-old transient, after Hill advanced on him and a partner with a knife with a 4-inch blade and then wound up to throw the weapon.

Crowell was working one of his last shifts at BART before a planned move to the FBI. The move was delayed, but Crowell's attorney, Harry Stern, said Thursday that the FBI had independently reviewed the shooting and determined it was justified. Crowell is now training to be a special agent.