When last seen James Earl Ray was walking impassively toward a doorway and back to prison, a slight, bespectacled, self-effacing figure carrying a briefcase. He looked more like a clerk in the accounting office than one of the century's most celebrated criminals.

Ray appeared on the Capitol Hill witness stand for three days, and then suddenly left. It will be months before he resumes his public testimony, and the manner of his departure only adds to the frustrations and controversies surrounding Martin Luther King's murder: nothing better underscores the differences between a trial and a widely publicized congressional hearing. Left hanging and festering now are major questions about his possible motives, the method of obtaining critical funds while a fugitive for months after month, and the heart of the controversy still surrounding the King assassination - was there a conspiracy, and if so, of what form and composed of what people, public or private?

Another question now clouds the House assassination committe's inquiry into King's death: the credibility of the committee itself.

That all this should be the unhappy result of the assassination committee's first week of public hearings probably comes as no surprise. The committee has been plagued with problems and discord from the beginning.

But the shame is that the committee had been proceeding with noteworthy dispatch and care until those final scenes ended the first act Friday. The hearings had not degenerated into a media circus, as widely feared. They were providing, contrary to much advance cynical speculation, a public service. They were permitting the American people for the first time to form impressions - and thus, in a sense, to judge - the character and personal story of James Earl Ray. There was hope that, out of further testimony, some of the conspiracy poison in the nation might be lanced.

Ray had been sticking to his story, but it was becoming increasing implausible under the persistent questioning and careful committee documentation. The tone of the hearings, thanks in large part to the patient, courteous handling of acting chairman Richardson Preyer who endured repeated insults and harangues from Ray's lawyer, Mark Lane, was judicious throughout. Then, just when the hearings were reaching a cumulative peak, the committee unloaded its most damaging assault on Ray himself and by implication, on all those who cling to complicated consipracy theories.

A lengthy deposition was read into the record. It present new - and startling - testimony from a British policeman who claimed to have had long conversations with Ray after Ray had been arrested in London in June of 1968.

The deposition was dramatic: Ray had told this policeman that he hated blacks and wanted to go to Africa and kill some more; that he was certain he would become a hero in the United States by killing a controversial black leader; that he could profit immensely from the crime by book and movie rights; that he craved publicity and was obsessed by what the press was reporting about him after his arrest; that he was clearly the "complete nut case."

Here, indeed, was strong material: motive, mental condition, attitude, all adding up to the classic portrait of the deranged, lone assassin. And all of this explosive stuff had just surfaced two weeks ago, thanks to the investigative work of the House committee and the conscience of the English cop. The policeman, identified as a retired detective chief inspector named Alexander Anthony Eist, had decided to come forward with this material after he ran into an unnamed American couple in a tavern who urged him to tell what he knew for the country's sake.

After this information was read into the record Mark Lane erupted in typical bellicose fashion. The policeman, he said he had just learned, was "possibly the most corrupt man in the history of Scotland Yard," one who had been thrown off the force and arrested in connection with jewel thefts and perjury.

The committee conceded it was checking into the policeman's background, and not necessarily accepting his story as true. Later, the word from England was that the policeman had been charged and tried in connection with jewel robberies and conspiracy "to pervert the course of justice." He was, it's reported, found not guilty. But for the House committee to put any credence in the sudden new testimony of a man with such a record without fully determining in advance his character, reputation and credibility casts serious doubts on all of its actions.

That effectively ended James Earl Ray's testimony.

What followed next only added to the sense of disbelief in everthing being testified - but the committee didn't seem to get the irony.

The next witness testified he had been treated in the psychiatric ward of a Veterans Administration hospital in Memphis. He was in Washington to confess he had committed a hoax on Mark Lane and The National Enquirer, a scandal sheet that feeds on conspiracy theories. Dean Cowden told Lane and the newspaper he had seen Ray at a gas station in Memphis at the same time that King was killed thereby giving Ray a clear alibi.

In fact, he was hundreds of miles away in Texas at the time. He had concocted the story at the urging of a Memphis private detective anxious to cash in on the King case with a book and movie deal. Mark Lane, he testified, fell for his story "hook, line and sinker."

Gales of laughter from committee members after hearing how crafty, big-time Mark Lane had been hoodwinked. They didn't seem to realize that they themselves might just have been conned by a former English cop now operating a pub, or that their own credibility was being called severly to account.

At the outset of the hearings, the courtly Richardson Preyer reminder the public: "This is not a trial. There are no prosecutors. There are no defendants." At the same time, some on the committee have hoped, privately, that their hearings would in fact provide the trial the American people had been denied after King's assassination 10 years ago.

In a sense, both thoughts are correct, and both present inevitable problems. The committee hearings are a trial, but one without the final conclusive ending.

They show us a repellant scene and strange specimens - the furtive, lifelong criminal lurking in shabby rooming houses, the con artist, the publicity hound, ther merchants who prey on tragedy for a shane of blood money. They cast light in dark areas, ask questions and raise new ones, offer a public service or contribute to further public cyncism. They don't tell the whole truth, but neither does any trial.

In the end they must reach some kind of verdict, and then find that they themselves have been on trial.