Film critic Roger Ebert seized on the photo of Dunn and tweeted hours after the accident: "Friends, don't let jackasses drink and drive."
His tweet touched off a firestorm.
Margera tweeted in response: "I just lost my best friend. I have been crying hysterical for a full day and piece of s**t Roger Ebert has the gall to put in his 2 cents."
Perez Hilton also took Ebert to task. "Everyone makes mistakes, and this is somebody's son," Hilton wrote on his website Monday. "Too soon, Roger."
On Tuesday, Ebert defended his remark while offering sympathy to Dunn's and Hartwell's families.
"I was probably too quick to tweet. That was unseemly," he wrote on his blog, adding that he regretted that his tweet was viewed as cruel.
But, he continued, "It was not intended as cruel. It was intended as true."
Dunn's passenger in the car, Hartwell, was not a member of the "Jackass" crew. But on IMDB he is listed as a production assistant on the second "Jackass" movie and is credited as "car stunt driver" on Bam Margera's 2009 movie "Minghags."