But the report has not been endorsed by the United Nations Assistance Mission in Afghanistan, which oversees UN operations in the country.
The attack against a parliamentary commission in the northern Afghan province of Baghlan on November 6 has been described as the deadliest suicide bomb attack in Afghan history.
Some 77 people were killed -- including six members of the Afghan parliament and scores of school children -- while more than 100 others were injured.
But the report by the UN's Safety and Security Department says it is believed that the lawmakers' bodyguards fired at least 100 bullets "deliberately and indiscriminately" into a crowd after the bomb blast. It also says that children who were greeting the parliamentary delegation at the opening of a new factory bore "the brunt of the onslaught at close range."
Both the Afghan government and the Afghan parliament have begun investigations into the suicide attack.
Zemary Bashari, a spokesman for the Afghan Interior Ministry, told RFE/RL's Radio Free Afghanistan today that the findings of the internal UN report differ significantly from what Afghan investigators have discovered.
Bashari says the Interior Ministry cannot confirm that anybody was shot and killed at the scene of the suicide bombing.
"The information we have obtained so far indicates that the bodyguards of the parliamentarians opened fire immediately after the explosion," Bashari said. "But the evidence shows that they fired into the air and did not target any individual. The holes which exist in the bodies of the killed and injured people were due to fragments [from the bomb]. Not bullets."
The chief spokesman for the United Nations Assistance Mission in Afghanistan, Adrian Edwards, told RFE/RL today that the internal UN report is only one of several conflicting views about what happened in Baghlan Province.
"We are still following the bombing in Baghlan and trying to establish from all the varying accounts of what has happened what the most likely causes of death were -- because it does appear that some people were killed by a gunshot and others by the bomb itself," Edwards said.
"Nobody at this stage has been able to establish a final version of events," Edwards said. "These accounts have not all been resolved and no final numbers have been arrived at."
Indeed, the UN security report says it is remains unclear how many people died from the suicide bombing and how many died from subsequent gunfire. But it says as many as two-thirds of those killed or injured appear to have been hit by gunfire.
And regardless of the exact figures in the case, the report says, "the fact remains that a number of armed men deliberately and indiscriminately fired into a crowd of unarmed civilians that posed no threat to them, causing multiple deaths and injuries."
Edwards said the UN is waiting for investigations by the Afghan government and parliament to reach their conclusions. He notes that the UN doesn't conduct its own direct investigations into such incidents, but instead monitors events involving civilian casualties and tries to verify the facts.
"In Afghanistan, there are several UN bodies which might look into an incident like this," Edwards said. "The Department of Safety and Security is primarily concerned with the safety of staff working for the UN. We also have a human-rights mandate to monitor civilian casualties. And we also work with our political affairs unit to try and establish the facts so that we understand what happened through them."
Edwards said no decision has been made about whether the UN will publicly release its final conclusions.