The sources said the pre-positioned militants, who posed as students, conducted 'extensive reconnaissance missions as a prelude to the attacks'.
'These eight men rented a house posing as Malaysian students, an intelligence source said on condition he was not named.
There were also believed to be other infiltrators who stockpiled weapons and ammunition, including in one of the two luxury hotels that were attacked.
They were joined on Wednesday evening, when the assault began, by a second group who reached Mumbai from the sea, the sources said.
The officials said the attackers were 'all well-built and at the peak of their health, aged between 24 and 30, and were heavily trained in military tactics'.
'For nourishment, they were well stocked with dried fruits, almonds and so on,' the source said, explaining how some of the militants managed to fight Indian commandos for 60 hours.
The intelligence sources declined to confirm or deny that the militants were either Pakistani nationals or had been trained in Pakistan, citing 'political sensitivities'.
India's Premier Manmohan Singh blamed the assault on Thursday on a group 'based outside the country and warned there would be 'a cost if suitable measures are not taken' to halt attacks.
Indian Foreign Minister Pranab Mukherjee has also said that 'some elements in Pakistan are responsible' for the assault.
Pakistan's government had fiercely denied it was in any way connected to the attackers.