Johnson refused to speak with reporters, a day after the Chiefs' 37-7 home loss to the Chargers. Sitting in front of his locker, he turned away and muttered the slur in telling reporters to get "out of here," according to the Star, which recorded the comment.
On Sunday, Johnson belittled rookie head coach Todd Haley on Twitter in a series of posts that also included the same gay slur.
Haley said Monday the Chiefs were looking into "a couple of situations" involving Johnson, but declined further comment. Asked about the slurs, he said the team was "gathering facts."
"I will say that how our players conduct themselves with the media and through the media is very important to myself as a head coach, and to the Chiefs as an organization," Haley said.
ESPN senior NFL analyst Chris Mortensen reported a league official said Johnson's slurs are "on our radar." The Chiefs are contemplating action against Johnson, league sources told Mortensen.
Johnson, drafted in the first round out of Penn State in 2003, needs only 75 yards rushing to overtake Priest Holmes' club record of 6,070 yards. But he is no stranger to controversy, having been benched for three games last year and suspended for one in connection with separate incidents involving women in bars.
The tweets were posted to the "Toonicon" account, an online alias the Star reported Johnson has said is his. The account includes a link to Johnson's personal Web site. Chiefs spokesman Bob Moore said the team could not verify that is Johnson's account.
The Twitter remarks were posted after San Diego routed the Chiefs (1-6).
One tweet read: "My father got more creditentials than most of these pro coaches." That was followed by: "My father played for the coach from "rememeber the titans". Our coach played golf. My father played for redskins briefley. Our coach. Nuthn."
Haley, hired earlier this year to replace the fired Herm Edwards, did not play high school or college football and attended college on a golf scholarship. He did not enter coaching until he was in his 20s. He was hired as Kansas City's head coach after serving as offensive coordinator for the Arizona Cardinals in their Super Bowl run last year.
His father, Dick Haley, is one of the NFL's most distinguished scouts and personnel men, having helped build the Steelers dynasty in the 1970s. Todd Haley has spoken of how he immersed himself in football while growing up and traveling with his father to games and on scouting trips.
"I'm very proud of what I've done to get to where I am," he said. "I'm very proud of my results as a position coach, as a coordinator. Right now, am I proud of my head coaching record? No. But I intend to do everything I can to change that."
Johnson's agent, Peter Schaffer, said, "I talked to Larry about it and he was focusing more on pumping up his father than anything else. He wasn't trying to downgrade anyone ... yes, everyone is frustrated after a loss like this but I wouldn't put too much into it."
The tweets drew responses, including one that referred to a nightclub incident in which Johnson was accused of spitting in someone's face.
Johnson's response included a three-letter gay slur. The final post read: "Make me regret it. Lmao. U don't stop my checks. Lmao. So 'tweet' away."
Johnson's account since has been made private.
A two-time Pro Bowler, Johnson was benched for three games by Edwards last year for an unspecified violation of team rules. NFL commissioner Roger Goodell suspended him for the fourth game for violating the league's player conduct policy in connection with incidents involving two women in bars.
Johnson then was sentenced to two years' probation after pleading guilty to two counts of disturbing the peace. One woman accused him of throwing a drink on her and another said he had pushed her head at a Kansas City night spot.
A judge earlier this year suspended the sentence and said he would remove the guilty plea from the public record if Johnson finished the probation period without further incidents.
He was one of the best running backs in the NFL in 2005 and '06, rushing for more than 1,700 yards in each season. In 2006, he set an NFL record with 410 carries, but his production fell to 559 yards the following season after he sat out training camp in a contract dispute.
Running behind a poor offensive line, he has not done well this season, averaging only 2.7 yards per carry.
Information from The Associated Press and ESPN.com's Bill Williamson was used in this report.