Kennedy was at his in-laws' home in Florida and collapsed about 1:15 a.m., Hillsborough County (Fla.) sheriff's spokeswoman Debbie Carter told the Associated Press. Kennedy's agent, Damon Lapa, told FoxSports.com that a brain aneurysm or heart attack are the suspected causes of death.
"It was definitely a shock to all of us," A's assistant general manager David Forst said. "Joe was here for close to two years, and he was certainly a fun guy to have around. He pitched well for us, and he was definitely a personality in the clubhouse."
Kennedy spent seven seasons in the majors, and was with the A's in three of them. He went 43-61 with a 4.79 ERA in 222 appearances with the A's, Toronto Blue Jays, Arizona Diamondbacks, Colorado Rockies and Tampa Bay Devil Rays. He went 11-15, 4.03 with the A's after they acquired him from the Colorado Rockies for Eric Byrnes in July 2005.
"It just seems unfair to take a guy like that from our world, because he meant a lot to so many people and he was so young," A's starter Dan Haren wrote in an e-mail. "It is such a tragedy and my heart goes out to all of his family."
Haren called Kennedy "one of my closest friends and teammates," and emphasized Kennedy's qualities as a "great dad, husband and teammate." Kennedy was married and had a 1-year-old son.
"He was just good people," former A's catcher Jason Kendall said. "I don't even know what to say. ... When you're with 25 guys more than you're with your wife and family throughout the year, (losing a teammate) is very hard."
Several members of the A's organization said Kennedy was a noticeable presence in the team's clubhouse, and not just because of his 6-foot-4, 252-pound frame. They said his personality stood out in a place where most players are simply willing to blend in quietly.
"There's so many different people that you meet in the game, but just his presence alone made him unique," reliever Alan Embree said. "He demanded attention just because of his presence. If you walked in a room, you knew Joe was there. He wore his emotions on his sleeve. If he was upset, you knew it. If he was happy, you knew it. So you kind of respect that."
The A's also respected the job Kennedy did for them for two seasons, the highlight of which was his work down the stretch in 2006. Kennedy, who missed half the season with shoulder tendinitis, didn't allow a run in his first 11 appearances upon his return, solidifying a bullpen that was a key component of the team's American League West title.
"He and (Justin Duchscherer) were like a left/right setup combination," Forst said. "... In '06, when he pitched, he was outstanding. He filled a real good need for us."
"He was a fierce competitor on the mound, always wanted the ball," A's manager Bob Geren said. "He had exactly what you wanted in a pitcher. He always wanted to be in the middle of the action as a starter or as a reliever. He always wanted to be in the tough games."
Kennedy started '07 as a starter but was moved to the bullpen after going 3-9 with a 4.37 ERA. He was claimed on waivers by the Arizona Diamondbacks in early August and later finished the season with the Toronto Blue Jays.
"We were terribly shocked," Blue Jays president Paul Godfrey told the AP. "From what we understand, he was in (Florida) ... to be the best man at a wedding today. Obviously when a 28-year-old man dies, ball player or not, it's a terrible, terrible thing."