Attorney General Janet Reno has asked former senator John C. Danforth, a Missouri Republican, to head an independent inquiry into the 1993 Waco siege, a Justice Department official said yesterday.

Danforth emerged as the leading candidate for the job last week. Since then, Reno and Danforth have had several discussions, which the official characterized as "exploratory" and "ongoing." Calls to Danforth's St. Louis law firm were not answered.

"This is not a done deal," the official cautioned. "She has called and said, 'Would you like to do this?' " The official said an announcement on Danforth, a three-term senator who retired in 1994, was probable today.

Reno decided to appoint an independent investigator last week, after revelations that the FBI had fired potentially incendiary military tear gas cartridges into the Branch Davidians' compound in Waco, Tex.--a claim the bureau had denied for six years in numerous congressional and internal investigations.

The new information, which Reno said she was not apprised of until a former FBI official's account was published in the Dallas Morning News last month, does not implicate the FBI in starting the fire that killed 75 Branch Davidians on April 19, 1993. It has succeeded, however, in raising questions about what other information the FBI may have withheld or misrepresented.

In addition to Reno's pledge to determine whether the FBI covered up facts about the 51-day standoff, numerous investigations of the Waco siege are once again underway. Justice Department officials are working to respond to subpoenas from the House Government Reform Committee, headed by Rep. Dan Burton (R-Ind.); a request for documents from Sen. Orrin G. Hatch (R-Utah), chairman of the Senate Judiciary Committee; and an order for all relevant documents from U.S. District Judge Walter Smith in Waco in connection with a case filed by survivors.