The Vikings might not have made one cannonball-type signing, but on the first day of the 2016 league year, they did plenty of splashing around in the free-agent pool.

After treading water during the two-day negotiation window, the reigning NFC North champs moved swiftly Wednesday to address two of their most significant needs.

The Vikings agreed to terms in the morning with former San Francisco 49ers guard Alex Boone. His four-year, $26.8 million deal became official minutes after the 3 p.m. opening whistle and includes $10 million guaranteed, according to a source.

Two hours later, they agreed to terms with safety Michael Griffin, who was released by the Tennessee Titans in February, on a one-year deal worth $2.5 million.

During the busy day, the Vikings also brought back guard Mike Harris and, per sources, hammered out deals with free-agent linebackers Emmanuel Lamur and Travis Lewis.

In all, they added four new players and made Harris their sixth re-signing to date.

Boone figures to be the most significant addition. The Vikings vowed to put a high priority on fixing the offensive line after it hamstrung the offense in 2015. No NFL quarterback was pressured more frequently than Teddy Bridgewater, according to Pro Football Focus. And while Adrian Peterson won his third rushing title, the Vikings feel the All-Pro running back too often had to try to make something out of nothing.

Boone, 28, is a two-time Pro Bowler who played 77 games with the 49ers, starting 59 of them, after going undrafted out of Ohio State in 2009. The 6-8, 300-pounder has played both guard spots for the 49ers. Boone lined up at left guard in 2015.

Boone should bring an edge to the line. Three years ago, he said he would like to punch Green Bay Packers outside linebacker Clay Matthews in the face after a particularly testy game.

Boone presumably will start at one of the two guard spots. Harris, who started at right guard in 2015, could compete with Brandon Fusco for the other guard spot or return to offensive tackle, the position he had played for his entire life until last summer.

Harris signed a one-year contract worth up to $3 million, according to a source.

His return means the team's starting five offensive linemen, including left tackle Matt Kalil, all will be back in 2016, and the Vikings are hoping for healthy returns from center John Sullivan and right tackle Phil Loadholt. Throw in Boone and perhaps an early draft pick, and the competition along the line figures to be fiercer.

The Vikings this week had expressed interest in Bengals free-agent safeties Reggie Nelson and George Iloka, who played for coach Mike Zimmer in Cincinnati. But they opted to sign Griffin, who has a history with defensive backs coach Jerry Gray. Gray was the defensive coordinator in Tennessee from 2011 to '13 before joining Zimmer's original staff in 2014.

Griffin, 31, was a first-round draft pick of the Titans in 2007. He played nine seasons with the Titans, starting 133 of 141 games. Griffin has tallied 763 career tackles, 25 interceptions and 11 forced fumbles. He was a second-team All-Pro in 2010 and was selected to the Pro Bowl in 2008 and 2010, but he has not sustained that high level of play.

The addition of Griffin could mean that 2015 starting safety Andrew Sendejo, who was re-signed to a four-year contract on Monday, shifts to a reserve role.

Lamur and Lewis will bolster the Vikings' depth at linebacker, where Chad Greenway, Jason Trusnik and Casey Matthews are free agents. However, Zimmer said two weeks ago at the NFL scouting combine that he expects Greenway to re-sign.

Lamur played in 39 games for the Bengals, starting 15. He has recorded 150 career tackles. At 6-4 and 245 pounds, he is similar in size to Pro Bowl strongside linebacker Anthony Barr. And he played a similar role under Zimmer in Cincinnati, too.

Lamur, who flew to the Twin Cities on Wednesday afternoon, will sign a two-year deal worth $6 million with $2.5 million guaranteed, according to multiple reports.

Lewis, meanwhile, started four games for the Detroit Lions last season and made 21 tackles. But he mostly contributed on special teams during his four seasons there.

The Vikings might not be done adding players. Reports linked them to Oakland Raiders wideout Andre Holmes, who spent the 2011 preseason with the Vikings, and James Hanna, who could potentially replace Rhett Ellison as the their blocking tight end if they can't reach an agreement to re-sign him.