A short summary of the House Committee on Assassinations:

.The HSCA completed its investigation in 1978 and issued its final report the following year, which concluded that Kennedy was probably assassinated as a result of a conspiracy. In addition to now-discredited acoustic analysis of a police channel dictabelt recording, the HSCA also commissioned numerous other scientific studies of assassination-related evidence that corroborate the Warren Commission's findings.

The HSCA determined, based on available evidence, that the probable conspiracy did not involve the governments of Cuba or the Soviet Union. The committee also stated that the conspiracy did not involve any organized crime group, anti-Castro group, nor the FBI, CIA, or Secret Service. The committee found that it could not exclude the possibility that individual members of the national syndicate of organized crime or anti-Castro Cubans were involved in a probable conspiracy to assassinate President Kennedy.[

In a memorandum written to the House Judiciary Committee in 1988 by Criminal Division Assistant Attorney General William F. Weld, the recommendations of the HSCA report were formally reviewed and a conclusion of active investigations was reported. In light of investigative reports from the FBI's Technical Services Division and the National Academy of Sciences Committee determining that "reliable acoustic data do not support a conclusion that there was a second gunman" in the Kennedy assassination, and that all investigative leads known to the Justice Department for both assassinations had been "exhaustively pursued", the Department concluded "that no persuasive evidence can be identified to support the theory of a conspiracy in either the assassination of President Kennedy or the assassination of Dr. King.

So basically,they based the 2nd shooter theory on evidence that we now know is B.S.
Also they claimed a probable conspiracy,except for the fact that it involved "somebody else" other than the usual conspiracy theory suspects.

Sounds like a crock of political knee-jerk bullshit.