Reporters Without Borders (RSF) said in a special report on Thursday that the Palestine Hotel killings were not a deliberate attack on the media.
However, it said US soldiers should have been told by their commanders that many journalists were based in the hotel.
Reuters cameraman Taras Protsyuk and Spanish television colleague Jose Couso were killed by the shell fired by a US tank in the Iraqi capital on 8 April.
"The shooting at this building was therefore not deliberately aimed at journalists or the press in Baghdad, as some people have claimed," the watchdog said.
"It was, however, an act of criminal negligence for which responsibility should clearly be established."
The report called for a reopening of a US army inquiry which ruled last August that American forces acted "in an appropriate manner" when they fired into the hotel.
RSF Secretary-General Robert Menard told a news conference that the US army's inquiry had done little more than excuse its own military of all responsibility.
He demanded more access to information on the death of the journalists and added: "We have the right to ask a big democracy to shed light on what happened."
The report criticised US authorities for maintaining before the inquiry that the reason for the firing of the shell was legitimate self-defence in response to shooting from the hotel.
"This first version of events became the official version and was a lie by the authorities," RSF said.
The report said the key to the tragedy was that soldiers in the field were never told that a large number of journalists were in the Palestine Hotel, nor was it marked on maps used by artillery support soldiers.
RSF says the Bush administration
must take some of the blame
"If they had known, they would never have fired," RSF said.
The report said Captain Philip Wolford, who gave permission for the shell to be fired at the hotel, and Sergeant Shawn Gibson, who fired the shell, were not responsible for the deaths.
But heavy responsibility should be borne at a higher level, at the headquarters of General Buford Blount, commander of the 3rd Infantry Division, "for not providing the necessary information that would have prevented the death of the journalists".
US government responsibility
RSF said since the US government had "supreme authority over its army in the field", it must also bear some responsibility for the shelling.
The tank shell hit a 15th floor balcony used by Reuters international news agency in the 17-storey hotel.
Reuters Ukrainian cameraman Protsyuk was wounded and died on arrival at a Baghdad hospital, and debris damaged the floor below, where cameraman Couso of the Spanish television company Telecinco was fatally wounded. Three other Reuters staff were also seriously hurt.
A spokeswoman for Reuters said: "The main conclusions in this report are generally consistent with the finding of our own investigation which identified a breakdown in communications between military commanders and troops on the ground.
"Since this attack, the safety of journalists in Iraq has not improved and we are taking every possible step, including actively engaging with the US military, to try and change this."