Since his 1979 conviction, Jeffrey MacDonald has painstakingly gathered a body of evidence

For the past 47 years, Jeffrey MacDonald’s account of what happened the night his entire family was murdered has stayed the same.

So has the case against him, built largely on blood-type testing — he, his wife and two children all had different blood types — that seems archaic by today’s standards.

But since his 1979 conviction, MacDonald has painstakingly gathered a body of evidence — some of which was suppressed by prosecutors — via Freedom of Information Act requests that he believes prove him right.

“I am innocent,” he says in an exclusive jailhouse interview in this week’s issue of PEOPLE magazine. “I did not murder my family. I have always told the truth about what happened that night.”

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On Feb. 17, 1970, MacDonald’s pregnant wife, Colette, and their two daughters, Kristen, 2, and Kimberley, 5, were slaughtered in their bedrooms.

While the Army cleared him of the crimes in 1970 after a six-week long Article 32 hearing, MacDonald, now 73, was convicted by a federal jury in 1979 and sentenced to three life terms in prison. On January 26, the 4th Circuit Court of Appeals will hear oral arguments on his “actual innocence” claim, a legal term that makes it exceedingly difficult to undo a conviction.

• For more on the Jeffrey MacDonald case, watch People Magazine Investigates: The Accused, on Jan. 24 at 8 p.m. and 11 p.m. ET on ID.

The new evidence includes: DNA testing of unidentified hairs found beneath Colette’s body, under Kristen’s fingernail and on her bedspread on the bed where she was murdered that do not match MacDonald; numerous confessions by police informant Helena Stoeckley and boyfriend Greg Mitchell, who are now deceased; and statements from now-deceased federal marshal Jimmy Britt, who says he heard then-prosecutor Jim Blackburn threaten to arrest Stoeckley if she testified at MacDonald’s trial that she was at the scene of the killings. (Blackburn has denied Britt’s allegation.)

Jeffrey MacDonald in January 2015

Other evidence his attorneys say was withheld from the defense at trial that points to intruders includes: wax drippings of three different types of candles found at the home that did not belong to any candles the MacDonald’s owned; long, blonde synthetic wig hairs found in a hairbrush next to the phone in their apartment; black wool fibers found on the mouth and bicep area of Colette and one of the murder weapons that were not matched to any fabric in the home; numerous unidentified fingerprints, palm prints and footprints at the crime scene that did not match MacDonald or his family, as well as other evidence MacDonald’s defense says was lost or destroyed due to what they say was the government’s “inept handling” of the crime scene.

Through the years, MacDonald has amassed some high-profile supporters, including some of the top forensic experts in the world.

“I believe that the story that Jeffrey MacDonald related that evening and ever since is correct,” says world-famous forensic pathologist Cyril Wecht, who has studied all the autopsy reports, physical evidence and trial testimony.

WATCH: Former Green Beret Surgeon Jeffrey MacDonald Says There’s Evidence He Didn’t Kill His Family: ‘I am Innocent’

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“The multiple and extensive injuries inflicted on Mrs. MacDonald and the two children and all the other things that were done at the house…depicts the involvement of two or more people,” Wecht says.

Jeffrey MacDonald
| Credit: AP

David Raskin, a nationally renowned polygraph expert who’s given thousands of lie detector tests, many of them in high-profile cases, says MacDonald passed a polygraph he gave him in March 1986.

“I asked him three questions about the murders,” says Raskin, a retired University of Utah psychologist. “The results indicated a definite ‘truthful.’ He got a score of 14 plus — 6 plus or higher is considered truthful. Everything I’ve learned since then [about the evidence] confirms that finding.”

Other prominent supporters include Barry Scheck, co-founder of The Innocence Project, the North Carolina Center on Actual Innocence, and C. Ronald Huff, a high-profile criminologist at the University of California, Irvine, who has been studying wrongful convictions for the past 30 years.

“I believe that Dr. MacDonald did not murder his family and that this will ultimately be regarded as one of the most egregious miscarriages of justice in the modern era,” says Huff.

US Attorney John Stuart Bruce declined to comment on the specifics of the case, saying in a statement to PEOPLE: “When cases are pending court proceedings, it is the practice of our office to litigate the case in court — through evidence and argument in hearings and in written filings with the court — rather than through the news media.”