Amnesty International has called on the Iranian authorities to release three US nationals who have been detained without charge or trial for a year.
Shane Michael Bauer, Joshua Felix Fattal and Sarah Emily Shourd were arrested by Iranian forces while they were hiking in the Iraq-Iran border area on 31 July 2009.
"One year on from their arrest it appears clear that the Iranian authorities do not have substantial grounds to prosecute these three individuals, and we fear that they may be held on account of their nationality," said Malcolm Smart, director of Amnesty International's Middle East and North Africa Programme.
"If so, they should be released immediately and allowed to leave Iran."
"If they are not to be freed, they must be charged with recognizably criminal offences and be tried according to international standards for a fair trial."
Iranian officials have alleged that the three planned to carry out "acts of espionage" in Iran. Their families and the US government deny this and the three have not been formally charged.
Iranian claims that the three were arrested after straying into Iran have been challenged by The Nation, an American weekly news publication, which said it had eyewitness testimony that they were seized in Iraq by Iranian Revolutionary Guards and taken forcibly into Iran.
Statements by senior Iranian leaders - including President Mahmoud Ahmadinejad in February 2010 - have suggested that the three may be being detained in order to put pressure on the US government and to extract diplomatic concessions.
"If this were the case, then the continuing detention of these three individuals would amount to hostage-taking and be a very serious abuse of human rights," said Malcolm Smart.
One year after their arrest, the Iranian authorities' failure to charge them with illegal entry into Iran or more serious charges, such as espionage, has fuelled speculation that the Iranian authorities are holding them as a bargaining chip.
"We believe that their questioning ended several months ago, so if serious charges were being considered these should have been brought by now."
The three are held at Tehran’s Evin Prison. They were allowed to telephone their families only several months after their arrest but in May 2010 they were taken to a Tehran hotel and allowed to meet their mothers who had travelled to Iran from the USA.
An Iranian lawyer appointed by their families to represent the three has not been given access to them and Swiss embassy officials, who represent US consular interests in Iran, have not been allowed to visit them since last April.
The families of two of the detainees, Sarah Shourd and Shane Bauer, say they have health problems which require regular monitoring.
"The detainees must be given immediate access to their lawyer, to renewed consular access and contact with their families, and to any medical attention or treatment that they need," said Malcolm Smart.
"The Iranian authorities must release these three US nationals without delay and allow them to leave Iran unless they are to face recognizable criminal charges and be tried promptly according to recognized international standards for fair trial."