Terrance Knighton has agreed to a one-year deal with the Redskins after a six-year career in Jacksonville and Denver. (Jack Dempsey/Associated Press)

Determined to upgrade a defensive line that grossly underperformed last season, the Washington Redskins on Thursday reached an agreement with former Denver Broncos nose tackle Terrance Knighton.

A seven-year veteran, the 6-foot-3, 331-pound Knighton agreed to a one-year deal worth $4 million, two people with knowledge of the situation confirmed.

Knighton’s addition follows the signing of Chicago’s Stephen Paea on Tuesday and the acquisition of Indianapolis’s Ricky Jean Francois two weeks ago.

The arrival of the trio follows the departures of nose tackle Barry Cofield and Stephen Bowen, both of whom battled injuries last season and had their contracts terminated last month, saving more than $15.7 million in cap space.

The team also opted not to re-sign 2011 second-round pick Jarvis Jenkins, making him a free agent. Jenkins never developed into the player the team hoped despite starting the better part of the past three seasons at right end.

NFL free agency opens Tuesday at 4 p.m., but the Redskins aren't expected to make any major moves. Post Sports Live debates whether there's a player the team could add who would make a significant impact this season. (Post Sports Live/The Washington Post)

Knighton, whose nickname is “Pot Roast,” spent the past two seasons in Denver. Last season, he recorded 30 tackles and two sacks, which followed a 31-tackle, three-sack season in 2013. A third-round pick out of Temple in 2009, Knighton played for the Jacksonville Jaguars for the first four seasons of his career.

But the Broncos failed to work out a deal this offseason to retain Knighton much to the dismay of his teammates.

When Knighton remained unsigned after the initial flurry of free agent activity, Denver’s DeMarcus Ware offered his support, tweeting “The fact that Pot Roast is a FA is freaking insane. Best Nose in League #facts.”

Knighton traveled to the Washington area Wednesday evening and dined with members of the Redskins organization and continued his visit at team headquarters on Thursday. He left without signing a deal, but his representatives and Washington brass finalized an agreement a few hours later.

In joining the Redskins, he reunites with Redskins defensive lineman Chris Baker, his teammate at Windsor High School in Milford, Conn.

When he learned of Washington’s serious interest in Knighton (who will serve as the best man in his wedding Saturday), Baker began campaigning on Twitter for the two sides to get the deal done.

After Knighton agreed to terms Baker tweeted “#PotRoast2DC it’s real the deal is done #SWAGGYTIME reunited [with] my dawg it’s going down #860 #HTTR.”

The Post Sports Live crew tries to figure out why Redskins head coach Jay Gruden would name Robert Griffin III as the team's starting quarterback in February. (Post Sports Live/The Washington Post)

Knighton, Paea and Knighton area expected to team with Baker, and Jason Hatcher (last year’s top free agent acquisition), among others, to anchor Washington’s 3-4 defense under new coordinator Joe Barry.

The team has yet to address the back end of the defense, which entered the offseason in nearly as bad shape as the defensive line.

Washington must replace both starting safeties, and also aims to add depth at cornerback. However, the Redskins efforts thus far have been unfruitful.

In the past two days, Washington has lost out on safety Antrel Rolle and cornerback Brandon Browner. Rolle signed with Chicago on Wednesday and told reporters that his final two teams came down to the Bears and Redskins. Browner was scheduled to visit the Redskins either Thursday or Friday, but agreed to a deal Thursday with New Orleans.

Meanwhile, two other quality safeties came off the market Thursday as Nate Allen and Rahim Moore signed with Oakland and Houston, respectively.

Washington hosted cornerback Perrish Cox on Wednesday, but he left without signing a deal and headed for a visit with Tennessee. Cox also has drawn interest from Cleveland.

Redskins officials still hope to add help in the secondary and along the offensive line among other places, but they are determined not to overspend, instead opting to find good value in free agency and build through the draft.

And speaking of the draft, General Manager Scot McCloughan was among a group of NFL talent evaluators who attended Oregon’s pro day to watch Heisman Trophy-winning quarterback Marcus Mariota and his teammates perform.

Tampa Bay GM Jason Licht, Titans GM Ruston Webster and Jets GM Mike Maccagnan, whose teams all hold top 10 picks, also were among observers.

The Bucs have the first pick and Tennessee drafts second. Washington holds the fifth pick, and the Jets the sixth. Jacksonville and Oakland, who both drafted quarterbacks last season, draft Nos.3 and 4.

McCloughan’s presence at Oregon only added to speculation over whether Washington would use its pick on Mariota (if available) despite having used the second overall selection on Robert Griffin III just four years ago.

McCloughan and Coach Jay Gruden have both said they need to upgrade Griffin’s supporting cast to give him a better chance to succeed.

Despite the Griffin’s struggles in 2013 and 2014, two people familiar with Washington’s offseason deliberations have described it as unlikely that the team would spend the fifth overall pick on a quarterback because of their abundant needs elsewhere on the roster.