New York (CNN) A Russian lawyer investigating an alleged $230 million money-laundering scheme nearly died in March after he fell from his fourth floor apartment in Moscow.

It's unclear what caused the fall, but more than a year earlier, American prosecutors had feared that someone would try to "threaten or harm" that man, Nikolai Gorokhov, according to a recently unsealed federal court document

American investigators are preparing for a trial in May that will examine how stolen Russian cash allegedly made its way into New York City real estate, and Gorokhov, 53, was to be a witness.

The court document shows that US investigators had "strong concerns that... individuals in Russia could attempt to threaten or harm Mr. Gorokhov and his family in an effort to prevent Mr. Gorokhov from testifying at trial."

The letter, dated Oct. 9, 2015, was written by former US Attorney Preet Bharara's team of prosecutors and addressed to a New York federal judge. It was unsealed April 14.

Attorneys for the accused Russian money launderers questioned Gorokhov for seven hours over two days in 2015. During that deposition, they served him with a subpoena that would force him to return and testify at trial.

American prosecutors asked the judge to not force Gorokhov to return, and to issue an order making clear that Gorokhov's already recorded deposition would fulfill all his testimony at trial. It was a move intended to "dramatically reduce the incentive for anyone in Russia hoping to influence this proceeding to threaten or injure him or his family," prosecutors wrote.

In her order, the federal judge granted prosecutors their wish.

The Department of Justice's Southern District of New York, which handled this case, declined to say why the document was unsealed -- a month after Gorokhov's near-fatal drop.

At the time of the fall, Russian media reported that Gorokhov was trying to winch a large bathtub up to his fourth floor apartment.

CNN visited the scene 24 hours later and a neighbor shared video shot with his cellphone shortly after the incident, showing an ambulance on scene and what looked like a pool of blood next to the upended bathtub.

Sergei Magnitsky, the independent Russian lawyer who initially exposed this massive alleged money laundering operation, died in a Russian prison in November 2009. Gorokhov, who has continued that private investigation on behalf of Magnitsky's family, is recovering from his fall, and has yet to speak publicly about what happened. Bharara, who initiated the American investigation into this scheme, was recently pushed out of office by US President Donald Trump.

Largest tax fraud in Russian history

This case goes back a decade.

According to US investigators, corrupt Russian government officials allegedly teamed up with a powerful Russian organized crime organization to pull off that country's largest ever tax fraud.

According to American investigators, in 2007, Russian police raided the offices of Hermitage Capital Management, at that time the largest portfolio investor in Russia.

Members of the Russian organized crime ring stole Hermitage's corporate identity, according to the U.S. federal complaint . The mobsters then created fake subsidiaries and had other corporate entities under their control launch multi-million dollar sham lawsuits against those subsidiaries, US prosecutors allege.

The "owners" of the fake subsidiaries then filed for tax refunds, citing their massive losses from the lawsuits.

Russian tax officials were allegedly in on it, and, in December 2007, approved the $230 million refund, the single largest in Russian history. It was processed in a mere 24 hours, the U.S. government charges.

The conspirators allegedly laundered the illicit cash through shell companies, luxury cars, real estate around the world, and international bank accounts.

Magnitsky was jailed on dubious charges of unrelated tax fraud when he exposed the $230 million scheme and its alleged perpetrators. According to US Senator John McCain, Magnitsky uncovered documents that "seem to suggest that Russian participants in the criminal activities... may include tax, police, and intelligence officials ."

The Russian Presidential Human Rights Commission has found that Magnitsky was violently assaulted in pretrial detention before his demise at age 37. The Russian government nevertheless insists that he died of a heart failure with no signs of violence.

Grave site of Sergei Magnitsky.

In 2011, Russian prosecutors put Magnitsky on trial posthumously for tax evasion. He was found guilty in the first criminal case ever brought against a corpse in that country's history.

In 2012, US Congress passed a landmark human rights law bearing his name, the Sergei Magnitsky Rule of Law Accountability Act, and sanctioned members of the Russian government who were allegedly involved in the massive mafia state conspiracy he uncovered.

In 2013, the US government filed criminal charges to seize New York City properties that were allegedly bought with some of these stolen funds. US prosecutors are chasing after three Manhattan apartments and more than a dozen bank accounts belonging to Prevezon Holdings and other shell companies that belong to Russians.

Prevezon denies the charges.

While investigating the fraud back in Russia, Gorokhov "photographed a Russian court file containing documents" that the US government plans to use at trial, according to the letter from the US Attorney's Office.