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Less than an hour before he killed three Baton Rouge officers, police assassin Gavin Long sent a handwritten note to at least one acquaintance in which he called the killings a “necessary evil” aimed at creating “substantial change in America’s police forces.”

In the message, which was emailed as three photo attachments, Long acknowledges that people who know him will be “in disbelief to hear from the media reports that I am suspected of committing such horrendous acts of violence.”

The email was time-stamped less than an hour before Long was first spotted by police carrying a gun and clad all in black. First reported by BuzzFeed News, the email's contents were confirmed to NBC News by a recipient, Yarima Kamara.

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Long refers in the message to an “unseen & concealed war within America’s police force between Good cops & Bad cops.”

“And the way the current system is set up, it protects all cops whether good or bad, right or wrong, instead of punishing bad cops & holding them accountable for their actions.”

Later in the letter, he says “Therefore I must bring the same destruction that bad cops continue to inflict upon my people, upon bad cops as well as good cops, in the hopes that the good cops (which are the majority) will be able to stand together and enact justice and punishment against bad cops b/c right now the police force & current judicial system is not doing so.”

Karama told NBC News he'd never met Long, but that he’d been familiar with Long’s online handle, Cosmo, because of comments on Karama’s YouTube videos. Karama said he didn’t know Long’s real name and didn’t put the Baton Rouge shooter together with “Cosmo” until news reports began to mention that name.

“I was like, 'Oh no,'” Karama told NBC News on Wednesday.

Karama said that representatives from the U.S. Department of Justice were on the way to his home to interview him about the letter. He declined to provide a copy to NBC News until after the meeting with authorities.

Despite that letter and other missives Long sent to various people in the days before the shooting, police believe his final actions were taken alone, a senior law enforcement official told NBC News Wednesday.

The official also says nothing has been found in Long’s electronic trail to indicate that he planned to attack police when he left Kansas City.

“He probably had mischief on his mind, because he took his guns with him, but it’s impossible to know what he was thinking,” the official said.