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Yes, thanks very much.

This new article has a much more useful summary of the alleged witness's claims:

The witness alleges slain DNC staffer Seth Rich was killed by two government employees, who worked as Rod Rosenstein’s “dirty squad” when he was U.S. attorney for Maryland, for breaching the Democratic National Committee server in 2016.
The witness, who called into a press conference using the alias ‘Luke,’ Tuesday, claimed Rich was murdered by three men that are his colleagues and friend, an AFT agent, a DEA agent and a former government employee.
He only learned of Seth Rich, he alleged, because the government agents were “bragging” they got away with killing him on a number occasions.
“The people involved in his death – I know them well, personally and professionally,” he told a room full of reporters on speaker phone at the Holiday Inn in Rosslyn, Virginia Tuesday.
“I do not know Seth Rich, never met him. Didn’t know the name until late 2016 when these people were bragging about what they’ve done and how they’ve gotten away with everything. This was through multiple conversations held with other people in front of me or in front of me and people I know,”
“I did some research and found out what they were talking about through my channels. They had details about what happened that night, leading up to it, the actual act, and occurred right after it and why,” he continued. “Whether or not he gave anything to WikiLeaks I have no idea. But going into the system, what they had control of set off everything. When he grabbed that many, they assumed it was for investigation.”
The witness explicitly detailed what how his colleagues allegedly murdered Rich after surveilling him for weeks.
“There were three involved that night. Two were waiting near his home. They had been watching him for a week or week and half to two weeks prior to this. They were waiting for hours. It was getting late – DC wakes up early. He was gone so long, they thought this would be sometime in the night,” he said. “They got the call, he was heading home. They went from being half asleep, to up on adrenaline.”
“They approached him from behind. The normal stop – ‘get on our knees, cross your ankles, hands behind your head.’ They couldn’t identify him, they wanted to make sure they had the right person. One came on him from behind, the other approached his right side. They had to get almost even with him because his arms were behind his head.” he continued. “They looked at him. They shot him in the ribs. He crawled to his left, he tried to scramble away – they ridiculed him in a sexual manner [about’ ‘getting on his knees and dying on his knees.’ He went forward, they shot him again.”
After shooting him, the alleged killers rolled him over, took the thumb drive and left.

Per the witness, Rosenstein has a conflict of interest in this matter.

Since it is almost certain that Sessions will hide behind his own recusal, someone other than Sessions or Rosenstein in the DOJ will need to be the supervisory authority.

It's not brain surgery what needs to happen. The FBI needs to corroborate statements of the witness to determine his reliability. Then, based on the reliability of the witness and corroborating evidence, investigatory tools requiring probable cause can be used against the suspects to determine if they in fact played any role in the murder of Seth Rich.

Perhaps the witness is peddling fiction, but it ought to be determined one way or the other.