Bell and Blount, the team's starter and backup at running back, were pulled over at about 1:30 p.m. ET Wednesday when the motorcycle officer smelled marijuana. Both admitted that the pot belonged to them.
Detective Brian Kohlhepp of the Ross Township (Pa.) Police Department told NFL.com that he expects marijuana possession charges to be filed. Bell could also be charged with driving under the influence.
Bell was driving a black Camaro, which included an unidentified female passenger. Police said all three were cooperative and released.
"We are aware of the reports and still gathering information," Steelers spokesman Burt Lauten said. "We will have no further comment at this time."
Once the case is resolved, Bell and Blount could face disciplinary action from the NFL under the league's substance abuse policy.
Bell, selected in the second round of the 2013 NFL Draft, rushed for 860 yards and eight touchdowns on 244 carries as a rookie last year.
Blount signed with the Steelers during the offseason after rushing for 772 yards and seven touchdowns on 153 carries last year with the New England Patriots.
The Steelers play in Philadelphia against the Eagles in a preseason game on Thursday night.