Tony Sparano, an NFL coach for nearly two decades, has died at age 56.

Twice a head coach with the Miami Dolphins and Oakland Raiders, Sparano had been on the NFL sideline for 18 years and was expected to enter another season as an assistant with the Minnesota Vikings when training camp began on Tuesday.

He was discovered unconscious at his home on Sunday morning, according to an ESPN report:

After starting his pro coaching career as an assistant with the Cleveland Browns in 1999, Sparano made stops in Washington, Jacksonville, Dallas, the New York Jets, Miami, Oakland and San Francisco before taking over offensive line duties in Minnesota in 2016.

“Our hearts go out to Jeanette and the entire Sparano family as we all mourn the loss of Tony,” Vikings owners Mark and Zygi Wilf said in a statement.

“Tony was a passionate and driven individual who cared deeply about his family, and especially enjoyed spending time with his grandchildren. Tony’s presence within the Vikings organization will be deeply missed. We are only thinking of Tony’s family during this incredibly difficult time. We ask that the entire NFL and Vikings family keep the Sparanos in their thoughts.”

Players from around the league reacted with sorrow as the news began spreading on social media.

A native of Connecticut, Sparano played center at the University of New Haven, where he began his coaching career in 1984, two years after graduating. He quickly rose through the college football ranks and became head coach of his alma mater in 1994, earning two playoff appearances in five seasons with the Division II school (41–14–1).

His next head coaching job came a decade later when the Dolphins introduced him as the successor to Cam Cameron by then-team vice president Bill Parcels in 2008. Miami was coming off a 1-15 season. He helped engineer the biggest turnaround in the NFL to date as Sparano led the team to an 11-5 record and an AFC East division title in his first season.

That dream run ended as the Dolphins fell to the Baltimore Ravens in the AFC wild-card game. It was Sparano’s only winning season with Miami and he was relieved of his duties during the 2011 season after a 4-9 start despite receiving a two-year contract extension weeks earlier.

Sparano was given a partial shot at redemption as interim head coach with the Raiders after the team started 0-4 in 2014, however Oakland was fielding a young roster and finished with just three wins on the year.

He was 32-42 overall as a head coach with the Dolphins and interim head coach with the Raiders.

As an offensive line coach with Minnesota, Sparano returned to the postseason as the Vikings won the NFC North and advance to the NFC championship game against a Philadelphia Eagles team that went on to win the Super Bowl.

Sparano is survived by his wife, Jeanette, his two sons and four grandchildren.

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Blake Schuster is a writer for Yahoo Sports. Have a tip? Email him at blakeschuster@yahoo.com or follow him on Twitter! Follow @Schustee