In an exclusive telephone interview with Aljazeera from Najaf, minutes after his release, Garen, 36, said he was grateful to al-Sadr's offices for securing his release.
He said he was seized in Nasiriya because of a misunderstanding. He had been taking pictures with a small camera in the souq (market) district of the city when he was overwhelmed by a large crowd.
"Nasiriya residents may have misunderstood my intentions and were offended," he said from an al-Sadr office as he awaited transfer to a "human rights organisation".
Aws al-Khafaji, an al-Sadr spokesperson, said Garen was released after al-Sadr personally intervened.
"Muqtada al-Sadr moved quickly after Garen's sister made a personal appeal to him for the release of her brother," al-Khafaji said.
"We asked for help to locate his captors during the Friday prayers sermon and then we received information that the group holding him was using his capture to apply pressure to US occupation troops who were besieging and attacking the holy shrines in Najaf."
Apparently, Garen was investigating the deaths of a family of Iraqis in Nasiriya who local sources said were killed when a US-led occupation forces opened fire on them.
Al-Khafaji added US military sources had said the car transporting the family was rigged with explosives.
"Mr Garen uncovered the truth about this massacred family," al-Khafaji said.
"He discovered that this was a family on its way to a hospital because they were expecting the birth of a baby."
The journalist and his Iraqi translator were seized in Nasiriya on Friday by a group calling itself the Martyrs Brigades.
It had threatened to kill him after 48 hours if US forces did not withdraw from Najaf.
Garen works for New York-based media production company Four Corners Media.