US authorities are investigating death threats sent to the parents of Bowe Bergdahl, the American soldier released by the Taliban last week after five years in captivity.

The FBI is examining four threatening emails sent to Bob Bergdahl and his wife Jani.

One of the emailed threats was sent to Bergdahl's father on the day a rally in their hometown of Hailey, Idaho, celebrating his release was cancelled abruptly, amid controversy over the circumstances of his handover to US commandos in Afghanistan on 31 May.

Meanwhile, more detail has emerged about the harsh conditions of his captivity, according to anonymous sources. Bergdahl, 28, has told doctors that he was locked in a metal cage in darkness for weeks at a time and was tortured and beaten by his captors, according to US officials quoted anonymously by the Associated Press and the New York Times.

Bergdahl was held in a metal cage as punishment for trying to escape, the New York Times reported, and is now being treated in a medical facility in Germany for skin and gum disorders typical of poor hygiene.

The freed soldier is said to be physically ready to fly back to the US, but officials reportedly remain concerned that he is not yet emotionally ready to cope with the pressures of reuniting with his family or handling the intense media exposure surrounding his case.

Controversy has raged over Bergdahl's release since it emerged that five Taliban prisoners in Guantánamo Bay were released in exchange. Critics of the deal have claimed that Bergdahl deserted his post and that six other US soldiers were killed in the initial efforts to find him.

It remains unclear how Bergdahl was captured nearly five years ago or why he walked off a remote outpost in eastern Afghanistan.

A welcome home party for the soldier was cancelled last Wednesday as the furore refused to die down, with Bergdahl's parents reportedly under police protection at their home.

Hailey's police chief, Jeff Gunter, told Reuters that the four threatening emails were being investigated by the FBI. FBI spokesman William Facer said in a statement: "The FBI continues to monitor the situation in Hailey, Idaho. We are working jointly with our state and local partners and taking each threat seriously."