photojournalist Luke Somers from al-Qaeda in Yemen were not aware of the identity of the other hostage held with him, a US official has told the BBC.
Both South African teacher Pierre Korkie and Mr Somers were shot by the militants during the raid, US officials say, and died as a result.
A charity working with Mr Korkie said he was to have been freed on Sunday.
Its statement said the US rescue attempt had "destroyed everything".
The US says the raid in southern Yemen was launched because they believed there was an immediate threat to Mr Somers' life.
However, it is understood that a payment had been made to enable the release of Mr Korkie.
Some workers at the charity that he was with, Gift of the Givers, have criticised the US raid.
A senior official in the US administration told the BBC's Tom Esslemont in Washington that the rescuers were not aware that Mr Korkie was being held with Mr Somers.
Separately, details have been emerging of the rescue mission. US officials, speaking on condition of anonymity, have said some 40 special forces took part in the rescue attempt, which came after drone strikes in the area.
The rescuers reportedly landed by helicopter around six miles (10 km) from the compound where the hostages were being held. Backed by Yemeni ground forces, they advanced to within 100m (320ft) of the site in Shabwa province.
A gunfight broke out when they were spotted by the militants, one of whom was briefly seen entering the compound. US officials believe this was when the hostages were shot.
Both hostages were evacuated with serious injuries and received immediate medical care. Mr Korkie died on a helicopter while Mr Somers died while being treated at US navy ship in the region.