Ray assassinated Dr. King.
US corporate media did not cover the trial, interview the King family, and textbooks omit this information. Journalist and author, James Douglass: 
"I can hardly believe the fact that, apart from the courtroom participants, only Memphis TV reporter Wendell Stacy and I attended from beginning to end this historic three-and-one-half week trial. Because of journalistic neglect scarcely anyone else in this land of ours even knows what went on in it. After critical testimony was given in the trial's second week before an almost empty gallery, Barbara Reis, U.S. correspondent for the Lisbon daily Publico who was there several days, turned to me and said, "Everything in the U.S. is the trial of the century. O.J. Simpson's trial was the trial of the century. Clinton's trial was the trial of the century. But this is the trial of the century, and who's here?" ”
For comparison, please consider the media coverage of O.J. Simpson’s trials: 
"Media coverage of the Simpson trial, which began in January 1995, was unlike any other. Over two thousand reporters covered the trial, and 80 miles of cable was required to allow nineteen television stations to cover the trial live to 91 percent of the American viewing audience. When the verdict was finally read on October 3, 1995, some 142 million people listened or watched. It seemed the nation stood still, divided along racial lines as to the defendant's guilt or innocence. During and after the trial, over eighty books were published about the event by most everyone involved in the Simpson case."
The overwhelming evidence of government complicity introduced and agreed as comprehensively valid by the jury includes the 111th Military Intelligence Group were sent to Dr. King's location, and that the usual police protection was pulled away just before the assassination.
Military Intelligence set-up photographers on a roof of a fire station with a clear view to Dr. King's balcony.
20th Special Forces Group had an 8-man sniper team at the assassination location on that day.
Memphis police ordered the scene where multiple witnesses reported as the source of shooting cut down of their bushes that would have hid a sniper team.
Along with sanitizing a crime scene, police abandoned investigative procedure to interview witnesses who lived by the scene of the shooting.
The King family believes the government's motivation to murder Dr. King was to prevent his imminent camp-in at Washington, D.C. until the Vietnam War was ended and those resources directed to end poverty and invest in US hard and soft infrastructure.
Please watch this six-minute video of the evidence from the trial,  and this eight-minute video  on the FBI's disclosures of covert operations against Dr. King, including confirmation from his closest friends and advisors.
Coretta Scott King, Dr. King's wife, is certain of the evidence after 30 years of consideration from the 1968 assassination to the 1999 trial:
"For a quarter of a century, Bill Pepper conducted an independent investigation of the assassination of Martin Luther King, Jr. He opened his files to our family, encouraged us to speak with the witnesses, and represented our family in the civil trial against the conspirators. The jury affirmed his findings, providing our family with a long-sought sense of closure and peace, which had been denied by official disinformation and cover- ups. Now the findings of his exhaustive investigation and additional revelations from the trial are presented in the pages of this important book. We recommend it highly to everyone who seeks the truth about Dr. King's assassination." — Coretta Scott King, Dr. King's wife.
The US Department of Justice issued a report in 2000 that explains their investigation into their own possible guilt in the assassination found no evidence to warrant further investigation.
Dr. King's son issued the following statement  rebuking a "self-study" rather than the independent investigation the King family assert the evidence demands:
"We learned only hours before the Justice Department press conference that they were releasing the report of their results of their 'limited investigation,' which covered only two areas of new evidence concerning the assassination of Dr. King. We had requested that we be given a copy of the report a few days in advance so that we might have had the opportunity to review it in detail. Since that courtesy was not extended to us, we are only able at this time to state the following:
1. We initially requested that a comprehensive investigation be conducted by a Truth and Reconciliation Commission, independent of the government, because we do not believe that, in such a politically-sensitive matter, the government is capable of investigating itself.
2. The type of independent investigation we sought was denied by the federal government. But in our view, it was carried out, in a Memphis courtroom, during a month-long trial by a jury of 12 American citizens who had no interest other than ascertaining the truth. (Kings v. Jowers)
3. After hearing and reviewing the extensive testimony and evidence, which had never before been tested under oath in a court of law, it took the Memphis jury only one (1) hour to find that a conspiracy to kill Dr. King did exist. Most significantly, this conspiracy involved agents of the governments of the City of Memphis, the state of Tennessee and the United States of America.
The overwhelming weight of the evidence also indicated that James Earl Ray was not the triggerman and, in fact, was an unknowing patsy.
4. We stand by that verdict and have no doubt that the truth about this terrible event has finally been revealed.
5. We urge all interested Americans to read the transcript of the trial on the King Center website and consider the evidence, so they can form their own unbiased conclusions.
Although we cooperated fully with this limited investigation, we never really expected that the government report would be any more objective than that which has resulted from any previous official investigation."
Let's summarize: Under US Civil Law, covert US government agencies were found guilty of the assassination of Dr. Martin Luther King, Jr.
Dr. King was the leading figure of the Civil Rights Movement, a Nobel Peace Prize winner, and widely recognized as one of the world’s greatest speakers for what it means to be human.
The family's conclusion as to motive was to prevent Dr. King from ending the Vietnam War because the government wanted to continue its ongoing covert and overt military operations to control foreign governments and their resources.
It is therefore a factual statement that under US Civil Law, the US government assassinated Dr. King.
This is similar that under Criminal Law, both O.J. Simpson and the US government are not legally guilty for murder, but both parties are guilty for killing innocent victims under Civil Law.
People of sufficient intellectual integrity and moral courage will embrace the trial evidence and testimony, jury conclusion, and King family analysis as appropriate and helpful information in seeking the facts.
69 Conversation with Emmanuel, comte de Las Cases (20 November 1816), Mémorial de Sainte Hélène, v. 4, p. 251.
70 The Martin Luther King Jr. Center. Civil Case: King Family versus Jowers. Transcript of closing statement: http://www.thekingcenter.org/civil-case-king-family-versus-jowers/ .
71 Probe Magazine. The Martin Luther King Conspiracy exposed in Memphis. Douglass, J.: http://ctka.net/pr500-king.html . Mr. Douglass took what he learned from the MLK trial and wrote a book on the explanation and evidence that similar covert US government factions assassinated President Kennedy: JFK and the unspeakable: why he died and why it matters. Reviews for your consideration here and here. Because the evidence for JFK being murdered by interests in our own government is so strong, AP US History teacher John Hankey created this DVD: Dark Legacy.
72 One of many analyses: law.jrank. Media - The O.j. Simpson Case: http://law.jrank.org/pages/12147/Media-O-J-Simpson-Case.html
73 Documentation from the King family, trial information, and video resources: Examiner.com. Martin Luther King assassinated by US government: MLK civil trial decision. Herman, C. Jan. 15, 2011: http://www.examiner.com/la-county-nonpartisan-in-los-angeles/mlk-assassinated-by-us-government-martin-luther-king-civil-1999-decision . For further documentation of evidence: What Really Happened: The Death of Martin Luther King: http://whatreallyhappened.com/RANCHO/POLITICS/MLK/mlk.html
74 RevolutionNewz. MSM blackout - the US govt executed Martin Luther King...Proven in US court, 1999: http://www.youtube.com/watch?v=k383kA7p7vs
75 Global Research. Martin Luther King Day: King family statement on the Justice Department’s “Limited Investigation” of the MLK assassination. Jan. 15, 2007
76 George Washington University. The National Security Archive. Rumsfeld’s Roadmap to Propaganda. Jan. 26, 2006: http://www.gwu.edu/~nsarchiv/NSAEBB/NSAEBB177/ . BBC News analysis: US plans to ‘fight the net’ revealed. Brookes, A. Jan. 27, 2006: http://news.bbc.co.uk/2/hi/americas/4655196.stm
77 History matters. Church Committee Interim report: Alleged assassination plots involving foreign leaders: http://www.history-matters.com/archive/contents/church/contents_church_reports_ir.htm