SANTA CLARA, Calif. — Two positive drug tests derailed Ahmad Brooks' highlight-crammed college career at Virginia. The linebacker also thinks an accusation of bad offseason behavior factored in the Cincinnati Bengals' decision to release him over the weekend.

But Brooks, who once leaped over the entire Virginia Tech line to make a sack that lives on in YouTube glory, has landed on his feet once more in San Francisco, even more determined to reach his lofty potential with the 49ers before he runs out of NFL chances.

"Everybody has to live through those teenage years and early 20s," the 24-year-old Brooks said today, still sweating after his first practice on the West Coast. "I'm pretty sure a lot of people have done dumb things at that age. ... Coach (Mike) Nolan, he told me what he expected out of me, pretty much gave me detailed rules. Since I've been in the NFL, I've been clean, so I'm all right."

Most NFL scouts thought Brooks was more than all right. At 6-foot-3, he's among the taller linebackers in the league, yet he has speed and power superior to most guys his size, who often get typecast as defensive ends. That combination of bulk in motion still makes him irresistible to the 49ers, who claimed him off waivers Sunday despite Nolan's long-standing vow to avoid problem players.

"I do believe we have the structure and the manpower and the people in place that can allow us take on someone that we feel, with some assistance, we can do the right thing with," Nolan said, citing the team chaplain, the medical staff and several outside advisers. "We haven't taken on a whole lot of guys with some history things. But at the same time, the ones we have, we've been pretty successful with them."

The NFL's benches and practice squads are full of can't-miss college linebackers who missed in the pros, yet still hope to live up to the expectations of fans and teams who salivate over their physical gifts. Dontarrious Thomas is just such a player — but the 49ers waived him to make room for Brooks.

Brooks wasn't surprised to be snatched up by the 49ers, who scouted him extensively before Cincinnati picked him in the 2006 supplemental draft. He heard San Francisco even made a trade offer for him last week, but was rebuffed.

And the Niners weren't scared away by Brooks' history of problems at Virginia, ending in his dismissal from the team by coach Al Groh, Nolan's former boss with the New York Jets. Brooks dropped about 20 pounds between his departure from the Cavaliers and his first NFL season, and it wasn't all by design.

"I was in a depressed situation coming out of college," Brooks said. "I wasn't happy with the way things were going. I was supposed to be a top draft pick coming out of college, and everything was messed up, so I was just doing some things I shouldn't have been doing."

He did well early in Cincinnati, even starting five games as a rookie before losing his job when the coaching staff decided he didn't understand enough of the defense. Last season ended after just two games when he injured his groin, and his absence contributed to his loss in the training-camp battle between backup linebackers.

Brooks describes his release as "devastating. You never think it's going to happen to you, and when it does, it feels like your heart's been broken."

But Brooks also didn't help himself in April when he was accused of punching a woman in the face in northern Kentucky. He knows the trouble-plagued Bengals weren't pleased, even if the incident was quashed.

"I didn't do anything wrong," Brooks said. "I just made a dumb decision, I guess. It all worked out. I didn't get charged, I don't have anything on my record."

Now coached by Hall of Fame linebacker Mike Singletary, Brooks is likely to be the backup to Patrick Willis, the NFL's defensive rookie of the year and Pro Bowl selection. Willis is having exactly the career Brooks always imagined for himself, and he still thinks it's not too late to get it.

"You always want to prove something to yourself, first of all, (but) to your family and your coaches, because they helped you get to this point," Brooks said. "You want to make them proud, too."