It's the investigative report the Kremlin never wanted the public to see.

Authored by slain opposition leader Boris Nemtsov and released posthumously Tuesday by fellow activists and journalists in Moscow, the report is titled simply: "Putin. War."

The document, Nemtsov's last in a series of reports on corruption in Putin's Russia, presents evidence showing Moscow's hand in the Ukraine crisis, the Russian president's reasons for fomenting the crisis, and the hefty human and financial costs of the ongoing war in eastern Ukraine, according to a copy obtained by Mashable.

Nemtsov was shot in the back and killed in the shadow of the Kremlin as he walked home with his Ukrainian girlfriend just before midnight in Moscow on Feb. 27. Investigators have detained five men in connection to Nemtsov's killing, all of them from Chechnya or other parts of Russia's predominantly Muslim North Caucasus.

Just days before his death, the former deputy prime minister had been working on what he promised would be a damning report on Moscow's direct military involvement in eastern Ukraine, where a bloody war has killed more than 6,000 people since April 2014.

Nemtsov's killing stoked fears that he was assassinated to prevent the 64-page report from getting out. After his death, investigators seized computers, hard drives, notebooks and other documents from Nemtsov's apartment.

Nemtsov rallies on the anniversary of the massive Bolotnaya Square protests one year earlier, on Dec. 5, 2012.

The Kremlin has repeatedly rebuffed charges that Russia is supplying weapons, military personnel and cash to separatists battling Ukrainian forces in the country's Donetsk and Luhansk regions. But its rebuttal has fallen apart as Russian soldiers returned to their motherland in body bags and others quit the military altogether to avoid being deployed to Ukraine.

Military convoys have been spotted driving across the Russian border into Ukraine by Western correspondents. Wounded troops have given vivid testimony to their involvement in hospital beds after being wounded in action.

Two boys watch as a Russian-backed rebel convoy rolls along the street of Stakhanov, eastern in Stakhanov, Ukraine, Friday, April 24, 2015.

The contents of Nemtsov's report

Nemtsov's report sheds little new light on Russia's military involvement in eastern Ukraine, as much of the material appears to have been obtained through open sources. But it does string into a narrative information and evidence gathered by activists and journalists over the course of the past 13 months.

The first chapter explores Putin's possible motivations for starting war in Ukraine. Nemtsov says it was all about the president's ratings.

In late 2011, Putin's poll numbers began to drop significantly, putting his chances of reclaiming the presidency at an all-time low. His victory was "only guaranteed by rigged elections," the report claims. "But even after reclaiming the throne he was on a downward spiral, pushing legislation that became increasingly repressive."

Russian President Vladimir Putin speaks at a rally marking the one year anniversary of the annexation of Ukraine's Crimea peninsula, outside the Kremlin, in Moscow, Russia, Wednesday, March 18, 2015.

The president's popularity continued to struggle. Then in 2014, following Kiev's Euromaidan revolution, Nemtsov claims Putin put Russia on a warpath, invading and annexation Ukraine's Crimean peninsula, and then trying to do the same in eastern Ukraine. That gave the president's ratings at home the stimulus he had needed to retain power.

Nemtsov spends the next chapters deconstructing how Russia seized Crimea. He scrutinizes the "lies and propaganda" of Russia's state media, which whipped up nationalist fervor in labeling Ukraine's new pro-Western government as "fascists and nazis" hell-bent on wiping out the country's Russian-speaking population, and examining the involvement of Russian regular forces and mercenaries in eastern Ukraine.

Captured Russian paratroopers are seen in this image taken from video in Kiev. Ukraine on Wednesday, Aug. 27. 2014.

In the chapter titled "Cargo-200," the Russian military code for the transport of dead soldiers, Nemtsov uses evidence gathered from previous public investigations to argue that conscripts are being sent to the front lines in Ukraine under the guise of volunteers.

According to his estimates, at least 150 Russian soldiers were killed in battle with Ukrainian forces near the town of Ilovaisk in August 2014. Relatives of those dead soldiers, according to the report, received 2 billion rubles, or almost $4 million, in compensation from the government — and were told not to disclose the circumstances of their loved ones' deaths.

Another 70 Russian troops died in the fight for Debaltseve in January and February of this year, says Nemtsov. Several of their relatives reached out to him after the state failed to fulfill its promise to compensate them for their family members' deaths and injuries, the late opposition leader adds.

Empty coffins are loaded into a refrigerated truck in Donetsk, Ukraine, May 29, 2014. The coffins were used to send the bodies of Russian citizens who were killed in fighting at Donetsk airport back to Russia.

In "Who shot down the Boeing?" Nemtsov gives credence to the theory that Russian forces or trained separatist proxies shot Malaysia Airlines Flight 17 out of the sky with a Russian "Buk" surface-to-air missile system over villages near Torez, eastern Ukraine, on July 17.

Russian authorities have floated their own theories; they first said Ukrainian SU-25 fighter jets opened fire on the passenger plane, but recently changed their story, saying a Ukrainian Buk rocket downed the jetliner.

Image: Pierre Crom, Getty Images

As further proof of direct Kremlin involvement in Ukraine, he discusses the players on the ground in Donetsk and Luhansk regions. They include Russian citizens, like Alexander Borodai, who first headed the "Donetsk People's Republic," and Igor "Strelkov" Girkin, the enigmatic commander who claimed to have personally "started the war in Ukraine."

Nemtsov's report also delves into the financial cost of the war, which is estimated to have cost about 53 billion rubles, or more than $1 billion — money that has come from the state budget. Additionally, the report says some 80 billion rubles, or $1.5 billion, has been spent by regional authorities on refugees from the Donetsk and Luhansk regions of Ukraine since July 2014.

'A cowardly and despicable war'

In conclusion, Nemtsov pleads to his Russian brethren for understanding.

"There is nobody in the world so near and dear to the Russian people as Ukrainians. They are our brothers," he writes.

The late opposition leader places the blame for the "cowardly and despicable war" on Putin himself, and warns those who support him: "It will cost our country."

"We will pay for this adventure with the lives of our soldiers, an economic crisis and political isolation," Nemtsov writes. The war, he says, "is a disgrace to our country," and "will not be resolved on its own."

"Putin must be stopped," Nemtsov writes. "And that can only be done by the Russian people ... Let's stop this war together."

Russians pay their respects to Boris Nemtsov at the place where he was killed in a gangland style shooting, Saturday, Feb. 28, 2015.

Nemtsov's report, unfinished at the time of his murder, was completed by Russian investigative journalists as well as activists of the political party Nemtsov co-founded, RPR-PARNAS. Together they pieced together what they managed to obtain from his home and office, including a handwritten note Nemtsov gave his assistant a day before his death.

Scribbled on paper so as not to be overheard by listening devices, and first published on Twitter by Nemtsov's colleague Ilya Yashin, the note reads: "Some paratroopers have been in touch with me. Seventeen killed, they didn't give them their money, but for now they are frightened to talk."

Yashin told RFE/RL that he felt it was his responsibility to publish his late mentor's work.

"When he was killed and I’d emerged from the initial shock, I realized it was my duty to the memory of my dead comrade to take this work to its end and publish the report that he began," Yashin said.

Speaking to journalists while presenting the report in Moscow on Tuesday, Yashin said money raised in publishing the report would be used to spread printed copies of it across Russia. "Even if 1% of Russians read this report, this will not have been in vain," he said.

The full "Putin. War" report, in Russian, is available to the public here.