Some witnesses who appeared before the grand jury investigating the death of 18-year-old Michael Brown were “clearly not telling the truth,” according to the St Louis county prosecutor, Robert McCulloch.



In his first public interview since announcing the grand jury’s decision not to indict officer Darren Wilson, McCulloch told local radio station KTRS that he had decided to put witnesses forward to testify regardless of their credibility.



“Early on, I decided that anyone who claimed to have witnessed anything would be presented to the grand jury,” he said.



“I knew somebody would be critical of whatever I did,” he said. “I thought it was important to present anybody and everybody, and some that were, yes, clearly not telling the truth, no question about it.”



The admission came just days after The Smoking Gun, an investigative site which publishes government, police and other documents, claimed to have identified a key grand jury witness and raised serious questions about the credibility of her testimony.

“Witness 40”, The Smoking Gun said, was Sandra McElroy, a 45-year-old St Louis resident who has a criminal record and a history of using racial epithets in online posts.

The site said it found her by using certain details in the unredacted portion of the grand jury transcript – Witness 40’s age, her home town, the fact that she was adopted, her two arrests for cheque fraud in 2007 – and checked them against public records and social media accounts.



In an update to its story, The Smoking Gun said on Tuesday that McElroy had confirmed her identity as “Witness 40” and was speaking to a lawyer.



The Smoking Gun also reported that McElroy came forward as a witness in another high-profile criminal case in 2007 – with a story that authorities later dismissed as “a complete fabrication.”

In her grand jury testimony, “Witness 40” described in detail how Michael Brown bent down “in a football position” and charged at officer Darren Wilson – an account often quoted in discussion of the case by rightwing commentators such as Fox News’s Sean Hannity.

It was noted in her testimony to the grand jury that “Witness 40” was not taking medication for bipolar disorder, and that she had memory problems after being injured in a 2001 car crash. The witness waited until after Wilson’s account of events was in the public domain, four weeks after the shooting, before coming to police with a story which directly corroborated his version of events.



The Smoking Gun also highlighted how “Witness 40” gave the grand jury several different explanations for her presence in Ferguson on the day Brown was killed – and that she admitted that her first version of the story was untrue.

McCulloch, in his interview, appeared to corroborate The Smoking Gun’s investigation of McElroy. “Some of the media is doing exactly what I said they would do,” he said. “They pull out one witness and just latch on to that.”

“This is a lady who clearly wasn’t present when this occurred, and she recounted this statement that was right out of the newspaper about Wilson’s actions, right down the line,” said McCulloch, “even though I’m sure she was nowhere near the place.”

“The thing that changed in her story several times was the reason she had to be in Canfield that day,” he continued.

The Smoking Gun alleged that McElroy’s testimony to the grand jury was also fabricated. “I believe there is a clear preponderance of evidence,” William Bastone, the editor of The Smoking Gun, told the Guardian. “We’d never have posted a story accusing her of perjury – and identifying her – if we weren’t 100% certain.”

This, too, appeared to be corroborated by McCulloch. “There are people who came in and, yes, absolutely lied under oath. Some lied to the FBI – even though they aren’t under oath, that’s another potential federal offence.” He added that he had allowed them to testify anyway because he had felt “it was much more important to present the entire picture”.

He said his department was not planning to pursue perjury charges.

The Smoking Gun reported that McElroy often posted online about the case after Michael Brown’s shooting, but before “Witness 40” went to the police with her story. In one reply to a post on Facebook about the case, McElroy said, “The report and autopsy are in so YES they were false.”

McElroy also reportedly regularly posted racial slurs about Brown in particular, and black people in general.

The Smoking Gun also reported that McElroy had previously been accused of lying about her involvement with a high-profile kidnapping case in St Louis. In that case, McElroy gave statements to the police in which she said that she had seen kidnapper Michael Devlin with victim Shawn Hornbeck months after his disappearance, and that she had known Devlin for 20 years. Police later declared those statements to be “a complete fabrication”.

Emails sent to several addresses believed to be connected to McElroy this week were not returned. On Wednesday, McElroy told the St Louis Post-Dispatch that she stood by her original story, and that she regretted ever coming forward. McCulloch was not available for comment on Friday.