One year ago, SCOTUS ruled in favor of the Manhattan DA to access Trump's tax returns. What's taking this investigation so long?

• It's been a year since SCOTUS ruled in favor of allowing prosecutors access to Trump's tax returns.
• Former Manhattan prosecutors say it isn't unusual for complex financial investigations to take years.

Exactly one year ago Tuesday, the US Supreme Court handed former President Donald Trump what was seen as a decisive loss — ruling that he cannot keep his tax returns secret from a Manhattan grand jury.

A year into crunching the numbers from eight years of Trump's corporate and personal taxes, prosecutors have yet to bring charges against the former president, something his lawyers have said demonstrates the weakness of the investigation.

But former Manhattan prosecutors told Insider that neither Trump's foes nor fans should read anything into how long things are taking.

"It's a huge undertaking, and it takes an incredible amount of time," said Diana Florence, a former prosecutor. "Add to that, it's a former president — it just makes it that much more complicated."

The offices of the Manhattan district attorney and the New York attorney general are conducting parallel investigations into Trump and his business.

The DA's investigation is nearly three years old and so far has resulted in limited tax charges against The Trump Organization and its chief financial officer, Allen Weisselberg.

At least 10 of Trump's properties are being investigated, according to court filings. Investigators say they have gathered some 5 million pages of Trump Organization documents, including the tax returns and supporting documents given to the DA after last year's Supreme Court ruling.

Sifting through that much data — and determining whether any crimes have been committed — is time-intensive work, former Manhattan prosecutors say.

In complex white-collar investigations, all documents must be transferred electronically into a keyword-searchable case database, said Florence, who handled complex enterprise-corruption cases as former chief of the DA's construction-fraud task force.

"But it's not computers that are ultimately looking at these documents," she added. "It's forensic accountants and experts. It's human beings at the end of the day. It's as much an art as it is a science. And all of this takes time."

Prosecutors must "go and verify each statement of fact in the return," said John Moscow, who also prosecuted complex economic crimes during his 30 years at the Manhattan DA's office.

"You don't say the return is false until you can prove that its constituent parts, taken together, are false," said Moscow, who is now senior counsel with Lewis Baach Kaufmann Middlemiss. "And that takes time."

Aside from the first charges against Weisselberg and Trump's company, no charges have been filed.

But Moscow and another former Manhattan prosecutor, Jeremy Saland, posited that — based on the public record so far — one possibility is that the AG and the DA could be readying a much more comprehensive case.

Lawyers for Trump and the Trump Organization have steadfastly denied wrongdoing.

Whatever Alvin Bragg, the Manhattan district attorney, and Attorney General Letitia James wind up doing — including not filing any new charges at all — the eyes of the world will be upon them, Saland said.

"Any mistake, even if it's superficial, will be ridiculed and exposed," he said. "They just have to make sure they dot their I's and cross their T's."