On Sunday, Kennedy's agent, Damon Lapa, was quoted as saying that an enlarged heart may have been the cause of death, according to doctors.
Results of Kennedy's autopsy are pending the completion of the tests, said forensic investigator Amanda Whidden on Saturday. She is with Hillsborough County's medical examiner's office.
"We are deeply saddened and shocked to hear of Joe's passing," A's assistant general manager David Forst said in a club release. "He was a valued teammate and friend to everyone with the A's organization. On behalf of the entire A's organization, we extend our condolences to Joe's wife, Jami, and his entire family."
Kennedy, who opened the 2007 season as Oakland's No. 5 starter but was dropped from the rotation and subsequently picked up on waivers by the Diamondbacks, closed the season with the Blue Jays, who signed him on Aug. 29 after he had been designated for assignment by Arizona.
"This is tragic and stunning news," Blue Jays GM J.P. Ricciardi said in a statement. "It is difficult to express the emotions felt by the Blue Jays players and our organization. My sincere sympathies go to his wife, their son and the entire family."
Kennedy was 43-61 with a 4.79 ERA in seven seasons with Tampa Bay, Colorado, Oakland, Arizona and Toronto.
News of the pitcher's death emerged Friday when Kennedy's agent, Damon Lapa, told ESPN.com that Kennedy died while at home. The Denver Post reported that, according to Lapa, Kennedy was visiting his wife's family in the Tampa area. He got up in the middle of the night and collapsed.
"The cause of death is unknown at this time," Lapa told FOXSports.com. "The best guess at this point is either a heart attack or a brain aneurysm. The way he died was sudden. There is no reasonable explanation for what happened."
Kennedy, an eighth-round pick of the Rays in 1998 out of Grossmont (Calif.) College, made his Major League debut in 2001 and was Tampa Bay's Opening Day starter in 2003.
"We are shocked and deeply saddened by the news of Joe's sudden passing and extend our deepest sympathies to his family," Rays director of Minor League operations Mitch Lukevics said in a statement. "Joe made all of us very proud as we watched him transform from a young pitching prospect in 1998 into our Opening Day starter in a very short time."
"He really dedicated himself and was really on a mission to become a major league pitcher," said Craig Weissmann, the Tampa Bay scout who signed Kennedy. "You wish as a scout and a Major League organization, you wish every kid could develop that fast."
Kennedy filed for free agency after the World Series, and Blue Jays president and chief executive officer Paul Godfrey told the AP that the club had "every intention" of speaking to Kennedy about returning to the Blue Jays next season.
"We were terribly shocked," Godfrey said. "From what we understand, he was in Brandon ... to be the best man at a wedding today."
Kennedy, acquired by Oakland at the All-Star break in 2005 and used almost exclusively in relief before being handed a spot in the A's rotation this spring, was moved back to the bullpen after going 3-9 with a 4.37 ERA as a starter this season. He appeared in 27 games, including 16 starts, before being placed on waivers.
The Diamondbacks claimed Kennedy on Aug. 4, but he appeared in just three games for Arizona, allowing seven runs in 2 2/3 innings, before being designated for assignment on Aug. 15. After being signed by the Blue Jays, he got his first win for Toronto on Sept. 21 against the New York Yankees.
Kennedy is the third active Major League player to die since October 2006. Cory Lidle, another former Athletic, pitched for seven teams from 1997-2006 before dying at age 34 when his small aircraft crashed into a Manhattan building on Oct. 11, 2006. Cardinals right-hander Josh Hancock, who previously pitched for the Red Sox, Phillies and Reds, died at age 29 in an auto accident on April 29, 2007.
Kennedy's best season as a starter was 2004, when he went 9-7 with a 3.66 ERA in 27 games with the Rockies. In 2006, he posted a 4-1 record and a 2.31 ERA in 39 relief appearances with the A's.
"Obviously, when a 28-year-old man dies, ballplayer or not, it's a terrible, terrible thing," Godfrey said.
Kennedy, his wife and their 1-year-old son, Kaige, lived in the Denver area, where Kennedy remained close friends with Rockies first baseman Todd Helton, who told the Denver Post he had recently played golf with his former teammate.
"They just bought a house," Helton told the Post. "This was the first Thanksgiving we haven't spent together in quite a while. This is just really sad news."
Rockies team president Keli McGregor extended his sympathies through a statement released by the team.
"Joe was a great husband, father, teammate and friend to so many in our organization and throughout the baseball world," McGregor said. "Our thoughts and prayers go out to his family, wife, his young son and all those whose lives were touched by Joe over his life."