The "suicide letter", and the FBI's covert activities (COINTELPRO) targeting the civil rights and Black Power movements more generally, came under significant scrutiny during the Senate's 1975 investigation into misconduct and abuse of power by the US intelligence community. The Church Committee's seven-volume final report — which was accompanied by a further six books of additional research and staff reports — is the most expansive exploration (and at times, an excoriating critique) of US intelligence agencies' capabilities and operations ever published.

A case study on the intelligence community's targeting of Martin Luther King in Book III of the report includes summaries of Sullivan's testimony regarding the letter. Speaking to the committee on 1 November 1975, Sullivan claims that former FBI Director J. Edgar Hoover ordered the letter's sending, via Associate Director Clyde Tolson, his longtime aide and most trusted lieutenant. The relevant section of the report:

William Sullivan, who was responsible for the project, testified that he first learned of the plan when Alan Belmont, Assistant to the Director. told him that Director Hoover wanted one of the King tapes mailed to Coretta King to precipitate their separation, therebv diminishing Dr. King’s stature. [...]

Sullivan told the Committee that he had opposed the plan because it would warn Dr. King that his activities were being covered by microphones. According to Sullivan, Belmont agreed that the plan was unwise, but said that he had no power to stop it because the orders had come from Hoover and Tolson.

Sullivan further denied having seen the document itself:

Sullivan testified that he did not recall ever having seen the document, although it was “possible” that he had something to do with it and simply cannot remember. Sullivan also testified that he could not recall any conversations at the FBI concerning the possibility of Dr. King’s committing suicide. After reading the last paragraph of the letter, he conceded that it could be interpreted as an invitation to suicide, although so far as Sullivan knew, the FBI’s goal was simply to convince Dr. King to resign from the SCLC, not to kill himself.

I haven't been able to find the original transcript of Sullivan's testimony. It's worth noting, though, that the two men he named as being responsible for the letter (Hoover and Tolson) were both dead by the time he was called to testify. Moreover, Sullivan was known to have despised Hoover, and had apparently become uncomfortable with how far COINTELPRO had gone. He was also widely believed to have coveted the directorship of the FBI. For whatever reason, in September 1971, he unsuccessfully tried to persuade the White House to fire Hoover; less than two weeks later, Hoover instead fired Sullivan. So, there was certainly no shortage of bad blood between the two.

In testimony on 19 November 1975, two other senior officials from Sullivan's Intelligence Division — Assistant Director Raymond Wannall and Deputy Associate Director James B. Adams — were also asked about the letter.

Senator Frank Church, the committee's chairman, quoted the section of the letter commonly interpreted as suggesting King's suicide, and asked Adams for his interpretation. Adams' initial response says very little:

I have read that statement. I have heard the conclusions of your staff that it was a suicide urging. I can't find any basis upon which they drew that conclusion. I think that, approaching it from an objective standpoint, as I read it, I don't know what it means. I think rather than a conclusion it should be a speculation in a realm of possibilities as to what was intended, but I cannot — I don't understand the basis for it. It is a possibility, but I certainly would not reach such a conclusion from my reading of that statement.

Church presses him further:

CHURCH: It reads, "You are done. There is but one way out for you." What does that mean?

ADAMS: I don't know. I don't know if it means confession. I don't know if it means suicide, as has been raised. I have no idea. You have the statement. I am not in a position to say. I haven't interviewed anyone that was with him at the time he received it.

CHURCH: Would you disown this statement and say that any connection the FBI had with it was utterly improper and grotesque?

ADAMS: I certainly would say it was improper, and I can't justify its being prepared or sent, yes, sir.

The Church Committee reports make very interesting reading indeed. The FBI have now published a large number of COINTELPRO files online. I'd recommend Tim Weiner's Enemies for a good history of the FBI. It's pretty dated now, but David Garrow's The FBI and Martin Luther King, Jr. may also be of interest.