(CNN) Five days after Jeffrey Epstein's suicide , federal Bureau of Prisons officials are struggling to establish even rudimentary facts of what happened over several key hours inside a prison rife with greater problems than previously known.

Many questions remain, among them:

What were the two guards who were supposed to monitor Epstein doing during the hours before he was found? Were they napping or did they simply fail to make the required checks every 30 minutes?

A confluence of missteps and what the Justice Department says were irregularities at the Manhattan Correctional Center have created a puzzle that FBI investigators are still trying to unravel.

Even top officials in the department have been frustrated by their inability to get some answers from the prison, in part because initial answers turned out to be inaccurate in some cases.

In one instance over the weekend, officials believed the former Epstein cellmate had been released on bail. But it turns out he had been moved to another facility, one person briefed on the matter said. One of the first tasks for FBI agents this week was interviewing that former cellmate, who could provide information on Epstein's behavior in the days before his suicide.

The Epstein debacle is arguably the biggest crisis so far for Attorney General William Barr and Deputy Attorney General Jeffrey Rosen, who have spent days overseeing the department's response. Next week the department faces a deadline from members of Congress, who have been demanding answers.

In a speech Monday, Barr described the Epstein failures in personal terms, saying he was "appalled" and "angry" at the "failure to adequately secure this prisoner."

He said the Epstein case was important to him -- for the first time noting that he had played a role in the decision to pursue a new federal case after federal prosecutors in another era had given Epstein a controversial plea deal

The FBI probe is complicated by the fact that key people involved aren't cooperating, people briefed on the matter say.

Eric Young, national president of the American Federation of Government Employees, which represents Bureau of Prisons employees, said employees should be given immunity from criminal prosecution.

"Our employee code of conduct policy requires all employees to cooperate during all official investigations, but that's contingent on whether (the FBI and the Justice Department inspector general) provide use immunity," Young said. "If (the employees) did something, whatever it is, even if it's illegal, that use immunity protects them, provided that they don't knowingly or don't willfully provide false statements."

He added: "Very few employees cooperate with a criminal investigation. If they make a statement that's contrary to evidence that's gathered they could be charged with a crime."

Young said the union doesn't control what the employees decide to do.

"The only time we get involved is when the employee asks for a representative," he said. "That employee is free to talk to whomever they want. They are free to do so. We do teach them to get the use immunity."

That means that even unlikely theories will require the time of FBI agents, who have to cross them off the list.

It also means, some officials fear, that when the FBI's findings are complete, there may still be lingering public doubts about what happened in a case that already has been the subject of outlandish conspiracy theories. It doesn't help that President Donald Trump helped spread some of those conspiracies

A Bureau of Prisons internal review this week includes a suicide reconstruction that officials say will look at everything from the first alleged suicide attempt last month to the decision to return Epstein to the Manhattan Correctional Center's special housing unit, where he died.

After the first incident, Bureau of Prisons officials determined that Epstein had tried to fake a suicide and they issued an infraction against him, which serves to put the event on his internal prison record, a person briefed on the matter said.