As Julian Assange emerged from his nine-day imprisonment, there were renewed concerns about the physical and psychological health of Bradley Manning, the former US intelligence analyst suspected of leaking the diplomatic cables at the centre of the storm.

Manning, who was arrested seven months ago, is being held at a military base in Virginia and faces a court martial and up to 52 years in prison for his alleged role in copying the cables.

His friends and supporters also claim they have been the target of extra-judicial harassment, intimidation and outright bribery by US government agents.

According to David House, a computer researcher from Boston who visits Manning twice a month, he is starting to deteriorate. "Over the last few weeks I have noticed a steady decline in his mental and physical wellbeing," he said. "His prolonged confinement in a solitary holding cell is unquestionably taking its toll on his intellect; his inability to exercise due to [prison] regulations has affected his physical appearance in a manner that suggests physical weakness."

Manning, House added, was no longer the characteristically brilliant man he had been, despite efforts to keep him intellectually engaged. He also disputed the authorities' claims that Manning was being kept in solitary for his own good.

"I initially believed that his time in solitary confinement was a decision made in the interests of his safety," he said. "As time passed and his suicide watch was lifted, to no effect, it became clear that his time in solitary – and his lack of a pillow, sheets, the freedom to exercise, or the ability to view televised current events – were enacted as a means of punishment rather than a means of safety."

House said many people were reluctant to talk about Manning's condition because of government harassment, including surveillance, warrantless computer seizures, and even bribes. "This has had such an intimidating effect that many are afraid to speak out on his behalf," House said.

Some friends report being followed extensively. Another computer expert said the army offered him cash to – in his words – "infiltrate" the WikiLeaks website. He said: "I turned them down. I don't want anything to do with this cloak and dagger stuff."

When the Washington Post tried to investigate the claim, an army criminal investigation division spokesman refused to comment. "We've got an ongoing investigation," he said. "We don't discuss our techniques and tactics."

On 3 November, House, 23, said he found customs agents waiting for him when he and his girlfriend returned to the US after a short holiday in Mexico. His bags were searched and two men identifying themselves as Homeland Security officials said they were being detained for questioning and would miss their connecting flight. The men seized all his electronic items and he was told to hand over all passwords and encryption keys – which he refused. The items have yet to be returned, said House. He added: "If Manning is convicted, it will be because his individual dedication to human ethics far surpasses that of the US government."

House, who met Manning through friends but came to know him only after his detention, said he was committed to his cause. "Like many computer scientists, I identify with the open government issues at the core of this case."