He called two black hecklers the "n-word" and enthusiastically referenced a time when blacks were often victims of civil rights abuses, but former "Seinfeld" star Michael Richards said his verbal barrage during a stand-up comedy routine was fueled by anger and not bigotry. "For me to be at a comedy club and flip out and say this crap, I'm deeply, deeply sorry," Richards said during a satellite appearance for David Letterman's "Late Show" in New York. "I'm not a racist. That's what's so insane about this," Richards said, his tone becoming angry and frustrated as he defended himself. Richards, best known for playing Jerry Seinfeld's eccentric neighbor Kramer on the hit TV show, described himself as going into "a rage" over the two audience members who interrupted his act Friday at the Laugh Factory in West Hollywood. His explanation was reminiscent of Mel Gibson's assertion that he wasn't anti-Semitic after he let off a barrage of Jewish slurs during a traffic stop this summer: despite what came out of his mouth, that's not what is inside him. Industry colleagues were in no hurry to accept Richards' apology. "Once the word comes out of your mouth and you don't happen to be African-American, then you have a whole lot of explaining," comedian Paul Rodriguez, who was at the Laugh Factory during Richards' performance, told CNN. "Freedom of speech has its limitations and I think Michael Richards found those limitations." Veteran publicist Michael Levine, whose clients have included comedians George Carlin, Sam Kinison and Rodney Dangerfield, called Richards' remarks inexcusable. Comics often face hecklers without losing their cool, he said. "I've never seen anything like this in my life," Levine said Monday. "I think it's a career ruiner for him. ... It's going to be a long road back for him, if at all." His Laugh Factory tirade began after the two clubgoers shouted at him that he wasn't funny. A videotape of the incident was posted on TMZ.com. Richards retorted: "Shut up! Fifty years ago we'd have you upside down with a f------ fork up your a--." He then paced across the stage taunting the men for interrupting his show, peppering his speech with racial slurs and profanities. "You can talk, you can talk, you're brave now mother------. Throw his a-- out. He's a n-----!" Richards shouts before repeating the racial epithet over and over again. Moderating his tone at one point, Richards tells the audience, "It shocks you, it shocks you" and refers to "what lays buried." While there is some chuckling in the audience throughout the outburst, someone can be heard gasping "Oh my God" and people respond with "ooh" after Richards uses the n-word. Eventually someone calls out: "It's not funny. That's why you're a reject, never had no shows, never had no movies. 'Seinfeld,' that's it." Jerry Seinfeld, who had issued a statement saying he was "sick over this horrible, horrible mistake" and calling it offensive, was scheduled as a Letterman guest Monday. He encouraged Richards to make a satellite appearance to talk about the incident, a CBS publicist said. Richards deserved the chance to apologize, Seinfeld said on "Late Show." "He's someone that I love and I know how shattered he is about" what happened, Seinfeld said. At one point, however, Richards grew flustered and expressed second thoughts about appearing on the program. "I'm hearing your audience laugh and I'm not even sure that this is where I should be addressing the situation," he said. Richards, 57, who played Kramer on the hit 1989-98 sitcom, hadn't spoken publicly about his remarks before "Late Show."