Seattle Seahawks cornerback Walter Thurmond is facing a four-game suspension for violating the NFL's policy and program for non-PED substances, a league source confirmed to ESPN NFL Insider Adam Schefter.

Thurmond is expected to begin serving his suspension, which was earlier reported by NFL.com, some time this coming week.

Thurmond's suspension would further deplete a Seattle secondary that already is without star cornerback Brandon Browner, who is expected to miss a month because of a groin injury.

The 26-year-old Thurmond has 31 tackles this season and played well last week in Browner's absence, returning an interception 29 yards for a touchdown in Seattle's victory over the Minnesota Vikings.

Thurmond took to Twitter on Sunday afternoon, acknowledging in a series of tweets that he "made a mistake" and that he is "disappointed."

I'm disappointed in myself for letting my teammates and family down. — walter thurmond III (@WaltThurm3) November 24, 2013

The Seahawks (10-1), who are on a bye this week, own the best record in the NFC but are closing in on the most difficult portion of their schedule.

The Seahawks will host the New Orleans Saints (9-2) on Monday night, Dec. 2, before visiting the rival San Francisco 49ers in Week 14 -- a pair of games that could determine whether Seattle secures home-field advantage in the NFC playoffs.