Rob Jackson’s interception against the Browns last season set up a go-ahead touchdown.

Redskins linebacker Rob Jackson is facing a four-game suspension to start the 2013 season because he tested positive for a banned substance,  two people with knowledge of the situation confirmed.

Jackson received word of the punishment before he signed a one-year deal with Washington earlier this month.

One source says the suspension was for the use of an unprescribed pain medication. The other source declined to say what substance Jackson tested positive for, but said that Jackson is appealing the punishment.

Jackson is coming off of a career year in which he started 14 out of 16 games and made 37 tackles, 4.5 sacks and four interceptions – one returned for a touchdown – and two forced fumbles.

Jackson moved into the starting lineup after Brian Orakpo tore a pectoral muscle in the second game of the season.

Update: The AP reported at 9:21 p.m. that Jackson has been suspended four games without pay for violating the NFL’s substance abuse policy. It also said the league said Wednesday night that Jackson will miss the first four games of the regular season.