"Michael Crabtree looked sharp catching passes for about 10 minutes Monday on the 49ers practice field.

As a result, the NFL is investigating whether the 49ers violated rules.

Crabtree was still on the physically-unable-to-perform list while catching approximately 25 short passes from 49ers quarterbacks, as well as a final throw from coach Jim Harbaugh.

General manager Trent Baalke eventually intervened and sent Crabtree away from the drill. That occurred after a 49ers public-relations official relayed a reporter's question to Baalke, asking if Crabtree's participation was allowed under PUP rules.

"We are looking into it," league spokesman Greg Aiello wrote in an email. "We appreciate Trent Baalke contacting our office to report what happened. Our understanding is that it occurred during warm-ups and that Michael Crabtree did not participate in practice."

Once Crabtree moved to a neighboring field to do individual conditioning drills, Baalke walked off the fields and into the team facility.

Baalke, promoted to general manager in January, was not made available for comment, nor was Harbaugh, who's in his first season as coach.

"The 49ers immediately notified the league office regarding the oversight that occurred earlier today when Michael Crabtree caught passes during warm-ups," a team spokesman said in a statement.

Crabtree, the 49ers' first-round draft pick in 2009, had been sidelined by a left-foot injury that occurred June 6 at player-organized workouts at San Jose State. He was put on the PUP list when training camp opened a month ago. The Sacramento Bee reported Sunday that Crabtree broke his foot June 6 and required surgery."