League sources told ESPN and NFL Media that Brown rejoined the Raiders at their practice facility and attended the team's final meeting before they broke training camp. Oakland will return to its home facility Tuesday in Alameda for practice before traveling to Winnipeg for Thursday night's preseason game against the Green Bay Packers.
Brown missed practice Sunday as he continues his ongoing battle to wear his old helmet, which is no longer certified for use. After his absence from camp, Raiders general manager Mike Mayock issued an ultimatum to the star wideout.
"You all know that A.B. is not here [Sunday], right?" Mayock told reporters. "So, here's the bottom line. He's upset about the helmet issue. We have supported that, we appreciate that. But at this point, we've pretty much exhausted all avenues of relief.
"So, from our perspective, it's time for him to be all-in or all-out, OK? So, we're hoping he's back soon. We've got 89 guys busting their tails, we are really excited about where this franchise is going and we hope A.B.'s going to be a big part of it, starting Week 1 against Denver."
Brown returned to training camp Aug. 13 and traveled with the Raiders to Arizona for the team's Thursday night preseason game against the Cardinals. He was also a full participant during Oakland's walk-through Saturday, with the team expecting him to practice fully Sunday.
The former Pittsburgh Steelers wideout temporarily ended his short absence from camp after finding a newer model of his old helmet. He was unable to wear his 10-year-old helmet because NOCSAE won't certify any helmet that was made over 10 years ago.
According to NFL Media, NOCSAE certified one specific Schutt Air Advantage helmet, made in 2011, for use, but the NFL told the Raiders the helmet would require additional testing before it could be used. The league informed Brown that the helmet failed the test, prompting him to skip Sunday's practice.
Brown's agent, Drew Rosenhaus, believes another option exists despite Mayock's comments. According to the NFL Network, two more helmets, made in 2014 and 2010, are being tested for possible use.
"He wore this helmet in Pee Wee football, high school football, college at Central Michigan and his entire nine-year career," Rosenhaus said. "People keep saying to me there's 2,000 players that are wearing approved helmets. But there are very few, if any, that have worn the same helmet their entire nine-year career.
"He's a wide receiver, he gets hit across the middle, people are trying to take his head off. He's a big target -- you talked about how many catches he's had over the last six years, for example. This is his life. He's risking everything. He's got a family. He's had a concussion before. This helmet has kept him safe."
Along with his ongoing helmet dispute, Brown has been hampered by frostbitten feet after failing to wear proper footwear while using a cryogenic therapy chamber. The All-Pro receiver was acquired in a trade with the Steelers this off-season.