The Paris-based media watchdog branded the US government's response to the incident as a "lie".
"The Reporters Without Borders enquiry shows there was lying, as well as three levels of responsibility," said the report, written by a journalist for the French weekly Le Nouvel Observateu on Thursday.
The 30-page document entitled Two Murders states that while US soldiers who fired the single tank shell that killed the two journalists and injured three others at the Palestine Hotel on 8 April could not be held accountable for the tragedy, their superiors did bear responsibility.
It said the enquiry showed top US military commanders stationed in Baghdad, as well as in the Gulf region and Washington never informed their troops on the ground the Palestine Hotel was full of journalists, and as such should not be targeted.
"It is inconceivable that the massive presence of journalists at the Palestine Hotel for three weeks prior to the shelling, which was known by any TV viewer and by the Pentagon itself, could have passed unnoticed.
"This presence was never mentioned to the troops on the ground or marked on the maps used by artillery support soldiers," the report said.
It added the unit that fired on the hotel did not know the building housed journalists and had it been informed so, it would have never fired.
Reporters carry a wounded
Reuters cameraman out of the
Reuters cameraman Taras Protsyuk and cameraman Jose Couso, of the Spanish TV station Telecinco, were killed and three other Reuters employees were injured when a single shot was fired into an upper floor of the hotel.
US soldiers involved in the incident said they had spotted a man with binoculars on one of the hotel's balconies, or on the roof, and mistook him for an enemy spotter directing the fire on US troops.
Sergeant Shawn Gibson, commander of the tank unit that fired at the hotel, was quoted as saying in the report had he known journalists were at the Palestine he would not have fired.
The watchdog criticised Washington for maintaining from the start the attack on the hotel was in legitimate self-defence despite evidence to the contrary.
"It is inconceivable that the massive presence of journalists at the Palestine Hotel for three weeks prior to the shelling, which was known by any TV viewer and by the Pentagon itself, could have passed unnoticed"
Reporters without Borders
"The first version of events became the official version and was a lie by the authorities," the report said.
"On two occasions, a month and then two months after the shooting,... (US Secretary of State Colin) Powell kept to the
original explanation, thus making it into a lie by the government."
It called for an investigation into the tragedy to be reopened in order to fully answer all outstanding questions.
"Since the so-called 'completed' US Army report on the killing of two journalists at the Palestine Hotel is not in fact complete, Reporters Without Borders demands the reopening of the enquiry to answer the real questions raised by their deaths."