ALAMEDA, Calif. -- Oakland Raiders receiver Antonio Brown, acquired in a trade this offseason following a drama-filled ending to his nine-year tenure with the Pittsburgh Steelers, was not at the Raiders' opening OTA practice on Tuesday.

The workouts are voluntary, and Brown had been present throughout the first phase of the offseason training program but left early last week to resolve his speeding case in a Pittsburgh court. He has not returned to the Raiders facility since, according to a source.

"He's not here today. Hopefully we'll see him here in the next couple of days. He's been working extremely hard, learning our offense. Really excited to get him out here. But in the meantime, we have plenty of balls to throw and plenty of receivers to throw to," coach Jon Gruden said Tuesday.

Brown has been active on his social media channels, showing numerous workouts with Raiders quarterback Derek Carr and his burgeoning relationship with him.

Conversely, Brown seemed to direct a barb at his former QB, Ben Roethlisberger, on Monday, tweeting "Two face" in the wake of Roethlisberger saying he may have been too tough on Brown when they were teammates.

Later, Brown tweeted: "Let's get back to work."