Kevin John Masterson, a 35-year-old mechanic wanted in connection with the Chicago Tylenol poisoning deaths, turned himself in at the FBI office here Monday and officials said he would be returned to Chicago Wednesday for questioning.

Soon after Masterson's surrender, however, outgoing Illinois Attorney General Tyrone C. Fahner said he was no longer particularly interested in questioning Masterson, the subject of a nationwide manhunt.

"I suspect," Fahner said today, "that he will turn out to be like so many other people that we've looked at, . . . who have simply said and done things which are . . . bizarre, . . . and that he's not the one responsible."

Fahner said he had been discussing Masterson's surrender with Masterson's attorney, David Schippers, since Nov. 22 and expected Masterson would take a lie-detector test.

Masterson, who had told friends he had a grudge against two of the retail outlets that sold the Extra-Strength Tylenol tainted with cyanide, was turned over to Los Angeles police by the FBI today to be held on an outstanding Illinois warrant for possession of marijuana.

Since seven persons died after taking the poisoned Tylenol between Sept. 29 and Oct. 1 in Chicago, a special task force has been attempting to follow up hundreds of leads, including tips over a telephone hot line. Such a tip initially brought Masterson to the attention of authorities.

In an affidavit filed when a search warrant was obtained for Masterson's home, Chicago investigators said that he and his wife, Joann, had once sued the Jewel Food Stores Inc. claiming his wife had been manhandled by employes in an alleged 1975 shoplifting incident. The suit was settled out of court, but according to Bob Fletcher of the Illinois state department of law enforcement, Masterson blamed the incident for the 1977 breakup of his marriage.

"He said he would get even with the store and a lot of people would get hurt," Fletcher said.

Another retail outlet that had drawn Masterson's ire was identified in an affidavit as Frank's Finer Foods, but the document did not elaborate the reason for Masterson's resentment against that store. Several bottles of poisoned Tylenol were planted at Frank's Finer Foods.

"Mr. Masterson is the kind of person, apparently, that says a lot of things which are not true, that he did not mean," Fahner said.

FBI spokesman John Hoos said Masterson appeared voluntarily at the FBI office in west Los Angeles at 3 p.m. Monday and was held overnight at the west Los Angeles division police jail on a federal warrant of unlawful flight to avoid prosecution. City police arrested him this morning and took him to the downtown Parker Center jail to await arraignment Wednesday morning, a police spokesman said.