On 31 July 2009, three US citizens - Shane Bauer, Sarah Shourd, and Joshua Fattal - were hiking in the mountainous northern Iraqi region of Khormal.
Their families say they crossed a poorly-demarcated border by mistake when they were detained by the Iranian authorities.
Officials in Tehran said they had "suspicious aims" and that they intended to put them on trial for espionage.
However, two Iraqis who witnessed the events, recently told the US magazine, The Nation, that uniformed Iranian border guards had crossed into Iraqi territory and detained them.
During a televised reunion at a Tehran hotel between the hikers and their mothers in May, Mr Bauer also denied that they had walked into Iran.
Ms Shourd was released on bail on 14 September on health grounds, but the two men remained in detention at Evin prison in Tehran.
US President Barack Obama welcomed Ms Shourd's release and said he remained hopeful that Tehran would demonstrate "renewed compassion" by freeing the other two Americans.
On 21 September, Bauer and Fattal were both released on bail set at $500,000 (£320,000) each.
Their families and US officials have always said the three - all graduates of the University of California, Berkeley - are completely innocent.
The 28-year-old was born and grew up in Onamia, Minnesota, with his mother, and in San Leandro, California, with his father. He is the eldest of three children.
A freelance journalist and photographer, after graduating he specialised in reporting on the Middle East and North Africa, where he spent much of his time. He is a Middle East correspondent for New America Media (NAM) and his work has been published widely.
The fluent Arabic speaker moved in 2008 to Damascus, where he lived with Sarah Shourd. The two met while organising demonstrations against the US-led invasion of Iraq. Mr Bauer has said he is opposed to "the US hegemonic policies in the Middle East".
After Mr Bauer's detention, Sandy Close, executive director of the Pacific News Service, said that she had employed him to cover the elections of the autonomous Kurdish region of northern Iraq. She said he had not intended to travel to Iran.
After seeing him briefly in May, Mr Bauer's mother said he was suffering from a severe stomach ailment.
It was also announced that Mr Bauer and Ms Shourd were engaged. He proposed in the exercise yard at Evin prison in January, his mother said. Josh Fattal will be the best man.
The 32-year-old, the youngest of three children, was born in Oak Park, Illinois, but grew up in Los Angeles, California.
A teacher, writer and women's rights activist, she lived in the San Francisco Bay Area after graduating from university until she moved to Damascus to live with Shane Bauer.
While in Syria, she worked for the Iraqi Student Project, a programme which seeks to make it possible for Iraqis to study at US universities.
She has written articles on travel and social issues reflecting her time in Syria, Ethiopia, Yemen and Mexico. In one article she recounted a meal as a guest of Iraqi refugees in Yemen, writing: "Everyday I feel ashamed at what my country has done to their country."
Ms Shourd had been held in solitary confinement at Evin prison, and her mother said she had been denied treatment for serious health problems, including a precancerous condition.
On 10 September, the Iranian judiciary briefly blocked a plan announced by President Mahmoud Ahmadinejad to release Ms Shourd at the end of the Islamic holy month of Ramadan as a good-will gesture.
Four days later, Ms Shourd was freed within hours of Iran's judicial authorities confirming they had received a $500,000 (£325,000) bail payment. She flew to the Gulf state of Oman, which was said to have played a key role in securing her freedom.
She could still be tried in absentia on spying charges.
The 28-year-old, the younger of two children, grew up in Elkins Park, Pennsylvania, but lived in Eugene, Oregon.
An environmentalist and teacher, he worked after graduation for three years at the Aprovecho Research Center in Cottage Grove, Oregon, a non-profit organisation that researches sustainable lifestyles.
From January to June 2009, Mr Fattal was a teaching assistant on the International Honors Program (IHP) Health and Community programme, which took him to Switzerland, India, China, and South Africa.
At the end of the semester, he travelled in Europe with his brother, who was conducting research in Sweden.
He travelled to Damascus in the summer of 2009 for a brief reunion with Shane Bauer and Sarah Shourd.
Mr Fattal was said to be undernourished and depressed in May, as were Mr Bauer and Ms Shourd.