This is an archived article and the information in the article may be outdated. Please look at the time stamp on the story to see when it was last updated.

SYCAMORE, Ill. — The Illinois State Police are looking into a possible new suspect in the 1957 abduction and murder of a 7-year-old Sycamore girl.

**For previous coverage, watch the video player above**

The disclosure, buried in an exhibit in a civil lawsuit, reveals an eye-popping twist: The new potential suspect emerged on the very same day the old suspect’s murder conviction began to unravel.

On that day — March 24, 2016 — a prosecutor named Richard Schmack informed a judge in Sycamore that an injustice had been done. He said the wrong man had been convicted in 2012 of the murder of Maria Ridulph. At the time, that conviction was believed to have solved the nation’s oldest cold case ever to go to trial.

The name of the new potential suspect, meanwhile, arrived in that afternoon’s mail. The tipster was shy, unlike others who came forward before with their suspicions in this case. The letter was neatly typed, but unsigned.

Schmack immediately turned it over to police.

Contacted by CNN, Schmack confirmed receiving the letter but declined further comment. A spokesman for the Illinois State Police declined comment because the matter is in litigation.

This stunning development in a case with no shortage of fusilli twists is revealed in an affidavit by Special Agent Ron Ogarek of the Illinois State Police. It was filed by attorneys for the state in response to a public-records lawsuit brought by a Seattle accountant, Casey Porter.

Porter’s father-in-law, Jack Daniel McCullough, was convicted in 2012 — and cleared earlier this year — of kidnapping and murdering Ridulph, his neighbor, on December 3, 1957. McCullough, who was 18 then and known as John Tessier, was questioned and cleared at the time by the FBI; he entered the US Air Force 11 days after Maria’s disappearance.

Porter, who maintains a blog about the case, said he sought the information to help clear McCullough’s name.

“I would like to thank Mr. Richard Schmack for passing along information related to a suspect in the Maria Ridulph case,” he said in an email. “I am glad the Illinois State Police finally acknowledge Jack McCullough is no longer a suspect.”

Since McCullough’s conviction was tossed, questions have arisen about whether police and prosecutors manipulated or withheld evidence while building the case against him. Schmack has asked for a special prosecutor to determine whether anyone engaged in misconduct.

Reached by email about the possible new suspect, McCullough observed that the police might be trying to portray themselves as heroes rather than waiting to be depicted as villains.

McCullough, who is 76, is asking a judge for a finding of innocence. His son-in-law’s lawsuit seeks police reports, emails and other records.

Ridulph, a second-grader, vanished while playing with a friend in the snow on a dark Sycamore street corner. A man who gave his name as Johnny approached the girls and offered them piggyback rides. Maria accepted, and disappeared with “Johnny” when the playmate ran home to fetch her mittens.

The Illinois State Police maintain that turning over reports, emails and field notes gathered during the McCullough investigation could hinder their new inquiry. The material sought also includes expense reports, travel receipts and communications about the case with other law enforcement agencies.

Ogarek’s affidavit states that although the Ridulph case was closed in 2012 with McCullough’s conviction, it was reopened earlier this year and police continue “to actively investigate this crime.”

He says the 123 pages of files Porter seeks contain personal and confidential information about potential witnesses, as well as details not known by the public.

Because the investigation spans nearly 60 years, the entire case file is voluminous. By some estimates, as many as 1,800 people were questioned over the years. A 1957 FBI memo reported that in just two weeks, the G-men had processed 200 suspects, “all with negative results,” and still had about 125 leads to go.

In its early stages, FBI chief J. Edgar Hoover and President Dwight Eisenhower closely followed the search for Ridulph. But the FBI dropped out as lead agency five months after her disappearance, when her body was found in a grove of trees along a highway near the Iowa border. The case went cold until 2008, when one of McCullough’s sisters called a police tip line and said their mother had pointed an accusing finger at him on her death bed, urging her daughters to “do something.”

Agent Ogarek spells out the steps he has taken since the investigation was re-opened: He studied the case file, completed “an investigative work-up of the individual named in the anonymous tip” and tried to locate that person.

He says the files related to witness Kathy Chapman, Maria’s childhood friend, are particularly crucial to the continuing investigation, and that releasing them could impair the inquiry.

“Kathy Chapman was the only witness to Ms. Ridulph’s disappearance,” the document states. “Therefore, the release of information contained in those pages could impair” the state police’s “current investigation into the March 24 anonymous tip, as well as future leads, by disseminating information that is currently confidential to the public.”

Chapman picked McCullough’s photo out of a lineup 52 years after Maria was abducted and continues to believe she correctly identified “Johnny.”

Additionally, the agent’s affidavit asserts that even if the current lead does not result in an arrest, disclosure of the information sought by Porter could affect state police investigators’ “ability to verify and discount future leads.”

Since it is impossible to predict when other leads might come in, “It is essential that the entire case file remain confidential” to protect investigators’ ability “to assess the validity of these potential leads.”