At the end of the season, he entered free agency.
He finished off the post with the hashtag #2ndAndFinalRetirement.
Harrison, who turns 40 next month, retired for the first time in August 2014, only to return to the Pittsburgh Steelers less than a month later after the team's defense suffered a number of injuries. There was some speculation the Patriots would sign him to another one-year contract in 2018, but it appears the 39-year-old is done for good. Harrison responded by saying he didn't want to merely collect a "participation trophy" and said "I didn't sign up to sit on the bench and be a cheerleader". He immediately went to the gym.
Harrison previously announced his retirement four years ago, only to reverse that decision.
UBC law professor questions Ottawa's jurisdiction on planned pipeline expansion
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That marked the second Lombardi Trophy of Harrison's career, having also triumphed with the Steelers three years previous against the Seattle Seahawks in Super Bowl XL.
In 14 seasons with Pittsburgh, Harrison won two Super Bowls and earned five Pro Bowl nods.
But it was the workouts that left people dumbfounded.
And there were pull-ups with added weight because his 250 pounds were not challenging enough.