By Andrew Seligman

AP Sports Writer

The Chicago Bears spent the offseason loading up to end a string of losing seasons. They just made their biggest move.

The Bears have acquired star pass rusher Khalil Mack from the Raiders in a massive trade that sends two first-round draft picks to Oakland.

Mack held out for the entire offseason and preseason, seeking a new, long-term contract rather than play under the final year of his rookie deal that would have earned him $13.8 million. Raiders coach Jon Gruden, in his biggest move since returning to the sideline from the broadcast booth, accommodated the 27-year-old Mack on Saturday.

Oakland will get first-round selections in 2019 and 2020, a sixth-rounder next year and a third-rounder in 2020. Oakland also included its second-round selection in 2020 and a conditional fifth-rounder that year.

Mack and the Bears agreed to a six-year, $141 million extension that guarantees $90 million. That makes him the highest paid defensive player in league history one day after Rams defensive tackle Aaron Donald, the 2017 Defensive Player of the Year, signed a six-year, $135 million deal, with $87 million guaranteed.

ESPN first reported the trade.

The fifth overall selection in the 2014 draft, Mack is a two-time All-Pro and the 2016 NFL Defensive Player of the Year. Mack has played in every game for Oakland the past four seasons, with 231 primary tackles, 40 1/2 sacks, and nine forced fumbles.

“We are excited to add a special playmaker like Khalil to our football team,” Bears general manager Ryan Pace said in a statement. “He brings a ton on the field, but he really fits what we are building in our locker room, too. Elite defensive players in their prime are rare so when we knew we had a legit shot to acquire him, we did everything we thought necessary to get him.”

Since entering the league, Mack leads all players with 185 1/2 quarterback pressures and ranks second with 68 tackles for loss, according to SportRadar. He is one of the most versatile defenders in the NFL.

Simply put, he’s a game changer.

The move sent shock waves around the league.

Raiders quarterback Derek Carr tweeted “No way” and added: “#RaiderNation we feel your pain trust me but we need you next Monday night!” Oakland linebacker Bruce Irvin made his feelings clear with a profane post. He later wrote, “What a shock. Now move on and win.. time to beat the Rams!!”

Mack had been a cornerstone in Oakland along with Carr ever since being drafted and helped the Raiders end a 13-year playoff drought in 2016. But he never formed a relationship with Gruden, who had said the opportunity to coach Mack was one of the reasons he was attracted to the job.

Gruden had called Mack Oakland’s best player but decided to go in a different direction rather than give another player a contract worth more than $20 million a year after Carr got a lucrative extension last summer.

The Raiders invested heavily in the pass rush in the draft, taking tackles P.J. Hall and Maurice Hurst, and end Arden Key in hopes of teaming them with Mack and Irvin. The three rookies showed promise in the preseason, but they will never get the chance to play with Mack.

Oakland hosts the Rams on Sept. 10. Chicago opens on Sept. 9 in a prime-time game against Green Bay.

A thrilled Bears left tackle Charles Leno Jr. tweeted a video of himself dancing to Mark Morrison’s “Return of the Mack.”

A three-time Pro Bowl pick, Mack is also the best pass rusher acquired by the Bears since they signed Julius Peppers away from Carolina with a six-year deal in 2010.

The Bears last traded two first-round picks when they acquired quarterback Jay Cutler from Denver in 2009. The deal for Mack is another bold move by Pace as he tries to lift a struggling franchise.

Chicago has four straight last-place finishes in the NFC North, five seasons in a row without a winning record, and just one playoff appearance since the 2006 team reached the Super Bowl.

But the Bears loaded up around prized quarterback Mitchell Trubisky, firing coach John Fox and replacing him with the offensive-minded Nagy. They added some new playmaking targets, most notably former Jacksonville Pro Bowl receiver Allen Robinson.

The Bears re-signed defensive coordinator Vic Fangio after passing on him for the head coaching job and drafted linebacker Roquan Smith with the No. 8 overall pick.

Now, the Monsters of the Midway are adding one of the NFL’s top pass rushers to a defense that ranked 10th overall and tied for seventh in sacks despite not having a Pro Bowl player. With his speed and power, Mack gives the Bears a huge threat on the edge to go with Leonard Floyd on the opposite side.