A Colts' teammate told Mortensen, "[Glenn] just feels burned out. It's that simple."
ESPN.com's Len Pasquarelli reported that Colts coach Tony Dungy had spoken to Glenn about his decision and there could be further discussions about with Glenn about his playing future.
Glenn has been a mainstay on the Colts' offensive line for 10 years and been a key to the excellent pass protection afforded quarterback Peyton Manning.
There had been speculation that Glenn, 31, and entering the final year of his contract, might be floating the retirement scenario as motivation for the Colts to offer a contract extension. But sources said the veteran offensive lineman was very serious in his deliberations about leaving the game as a Super Bowl champion.
Glenn is scheduled to earn a base salary of $4.5 million in 2007.
Glenn's departure would create a huge void for the Colts just a week before the start of training camp. There are no other accomplished left tackles on the Indianapolis roster and few quality veterans remaining among the free agents who are still looking for work.
Indianapolis chose Tony Ugoh in the second round of this year's draft, sending their 2008 first-round pick to San Francisco to move into position to select the former Arkansas star. But it is not believed he is ready to start. In fact, the plan was to use Ugoh primarily at guard this season, and groom him to eventually replace Glenn, perhaps as early as 2008.
Second-year veteran Charles Johnson, a sixth-round choice in 2006, has played some at left tackle, but lined up more often on the right side. Starting right tackle Ryan Diem struggles at times in pass protection, so switching him to the left side likely would not be an option.
There were apparently no indications earlier in the spring that Glenn, the Colts' first-round choice in the 1997 draft, was thinking of retiring. He participated in the offseason program and was at the team's mandatory minicamp.
A former University of California star, Glenn has appeared in 154 games for the Colts, all as a starter. The last 138 of those starts all came at left tackle. Glenn has missed just six games in his career, all in 2003.
Len Pasquarelli is a senior NFL writer for ESPN.com. Television reporter Chris Mortensen covers the NFL for ESPN.