When special prosecutor Dan Webb’s report left so many unanswered questions about the police investigation into the death of David Koschman, I thought we’d lost the last best chance of getting to the bottom of what happened.

Maybe not.

It turns out Webb received permission last summer to share the fruits of his grand jury investigation with the FBI, Sun-Times reporters disclosed Tuesday.

The reporters obtained documents showing Webb explained to a Cook County judge overseeing the grand jury that his purpose in turning over evidence was to get the FBI’s assistance with his case.

In doing so, however, Webb also opened the door for the FBI to launch an investigation of its own — and even suggested an unspecified investigative body already was looking into similar allegations involving the same police officers.

To be clear, we don’t know if there is a federal investigation into how police handled Koschman’s death — or mishandled it, as the evidence shows.

But it’s plain to me at this point there should be. A federal investigation is the next logical step in this sad saga. After coming this far, we can’t just let it drop.

This is no longer about Richard “R.J.” Vanecko, who is now serving a 60-day jail sentence in McHenry County for his role in Koschman’s case. It hasn’t been about Vanecko for a long time.

It’s about what may have been done to protect him from prosecution by individuals within the Chicago Police Department — first in 2004 and again in 2011 when the Sun-Times forced a re-examination of the case.

Webb and his team did an excellent job of identifying the weaknesses in the Chicago police investigation, but ultimately they were unable to answer the most troubling questions they raised.

For instance, why did Chicago police supervisors invent a self-defense claim for Vanecko, even going so far as to justify it with a threatening quote attributed to Koschman that doesn’t actually appear in any witness report?

Then there’s the strange circumstance of the police lieutenant who says he discovered the long-lost Koschman case file — and then took it home for a few weeks for safekeeping.

And don’t forget the troubling business of the top aide to Mayor Richard M. Daley, Vanecko’s uncle, who says he briefed the mayor on his nephew’s alleged involvement in Koschman’s death within days of the occurrence while detectives working the case say they didn’t learn Vanecko was involved until weeks later.

These matters go to the integrity of the Chicago Police Department, and they won’t go away, not until every resource has been brought to bear and every avenue has been explored.

It’s possible, of course, the investigation has simply run its course. After all, Webb’s team was composed of experienced federal prosecutors working with city Inspector General Joe Ferguson and his investigators, also former feds. Even the best investigators can’t always get to the truth of a situation.

I was probably leaning in that direction immediately after reading the Webb report, mainly out of discouragement.

But there’s a lot of meat in that report, as reporters Tim Novak and Chris Fusco have shown by serializing its contents in their follow-up stories.

With the longer reach of federal law and federal resources, it seems possible that new U.S. Attorney Zach Fardon could break through the wall that Webb hit.

One way federal prosecutors might do this is to bring before a federal grand jury some of the six police officers Webb considered bringing charges against, then squeeze.

That might require giving some of them immunity from prosecution. Such grants of immunity only apply if the witnesses tell the truth. If they don’t, they expose themselves to perjury or obstruction charges.

There are other scenarios that might keep the Koschman case alive.

Attorneys for Nanci Koschman, the victim’s mother, could file a federal civil rights lawsuit against the Chicago Police Department, as those same lawyers did in the police torture case of Jon Burge. Or the city inspector general’s office might yet issue a report on its own findings, separate from its work for the special prosecutor.

But until the feds weigh in, one way or another, this case will be impossible to put to rest.

Email: markbrown@suntimes.com

Twitter: @MarkBrownCST