MIDLAND CITY, Alabama (CNN) -- The rescue of a kindergartner from the underground bunker where he had been held for almost a week was precipitated by concerns the kidnapper's mental state was in sudden decline, a law enforcement source close to the investigation said Tuesday.

Authorities have said little publicly about the Monday rescue that killed 65-year-old Jimmy Lee Dykes and freed his 5-year-old captive just two days before the boy's birthday.

But the law enforcement source said Dykes' mental state deteriorated in the 24 hours before the Monday afternoon rescue, and experts from FBI units, including a crisis negotiation team, tactical intelligence officers and a behavioral sciences unit, determined he was in a downward psychological spiral.

The FBI's hostage rescue team forced its way into the bunker and rescued the boy, the source said.

While authorities have not said whether Dykes killed himself or if the team that stormed the bunker shot him, the FBI is sending a "shooting review board" from Washington to look into the incident, FBI spokesman Jason Pack said Tuesday from Alabama.

Evidence teams are also at the site, waiting for bomb technicians to finish their work, which was set to resume Tuesday morning, Pack said.

Dale County Sheriff Wally Olson said Tuesday he could not release much information about the case.

"It's still actually an ongoing investigation, and we still have a lot of work to do here," he said.

The boy, identified by authorities only as Ethan, remained hospitalized Tuesday, according to Midland City Elementary School Principal Phillip Parker. There is no time frame for his release, Parker said.

Monday night, the boy's uncle said he expected Ethan to remain in the hospital at least overnight.

FBI Special Agent in Charge Steve Richardson said Monday that Ethan was in a private area with heavy security.

"He is doing fine," said Richardson, who had visited the boy. "He's laughing, joking, playing, eating."

What's next for Ethan?

Relief that Ethan was safe was palpable, but many questions remain about what comes next for him.

How does a 5-year-old heal from this ordeal? How does a youngster go on after witnessing his bus driver shot to death, then being dragged to an underground bunker by a gun-toting stranger? How will he deal with what he experienced the six days he languished in that hole and what he saw during the explosive rescue Monday that killed his captor?

"It's very hard to tell how he's going to do," said Louis Krouse, a psychiatrist at Chicago's Rush Medical Center. "On the one hand, he might get right back to his routine and do absolutely fine. But on the other hand, the anxieties, the trauma, what we call an acute stress disorder, even post-traumatic stress symptoms, can occur."

The ordeal

Dykes boarded a Dale County school bus a week ago, demanding that the driver hand over two children, police said.

The driver, Charles Albert Poland Jr., refused, blocking access to the bus's narrow aisle as at least 21 children escaped out of the back emergency door, authorities said.

The gunman killed Poland, then grabbed Ethan before barricading himself and the boy inside a nearby bunker he had built on his southeast Alabama land. In the ensuing days, officials said little about what was going on in the bunker or in their strategy, or what -- if anything -- Dykes wanted.

While authorities have not said how they communicated with Dykes, they have said they were frequently in touch with him.

At one point Monday, Olson told reporters that Dykes had "a story that's important to him, although it's very complex."

Negotiations rapidly deteriorated, Olson said later. Dykes had also been observed holding a gun, according to authorities.

At 3:12 p.m. (4:12 ET), the FBI team went in.

One neighbor said he was outside when he was startled by the sound of an explosion.

"I heard a big boom and then ... I believe I heard rifle shots," said Bryon Martin, who owns a home near the bunker where Ethan had been held since last Tuesday.

It was a loud noise that "made me jump off the ground," he said.

Authorities wouldn't say whether the blast was set off as a diversionary tactic or whether Dykes had planted explosives around the bunker.

When the rescue was over, Dykes was dead and Ethan was unharmed.

Olson declined to say whether the boy saw his abductor die.

"He's a very special child. He's been through a lot, he's endured a lot," he said.

The recovery

Someone who knows all too well what Ethan may go through is Katie Beers, who as a 10-year-old was held underground in a concrete bunker for two weeks by a New York man.

"I am ecstatic that Ethan has been retrieved safe and sound," said Beers, who recently released a book about her abduction. "As for my ordeal, I just keep thinking about the effects of it: being deprived sunlight, nutritious food and human contact. And how much I wanted to have a nutritious meal, see my family."

Beers says she still feels the effects of her kidnapping.

"The major issue that I have is control issues with my kids and finances," she said. "I don't like my kids being out of my sight for more than two seconds. And I think that that might get worse as they get older."

Martin Savidge reported from Midland City; Lateef Mungin reported and wrote from Atlanta. CNN's Victor Blackwell, Vivian Kuo, Rich Phillips, Larry Shaughnessy, Barbara Starr, Michael Pearson and Steve Almasy and HLN law enforcement analyst Mike Brooks also contributed to this report.