Not in what you posted. Read it and see for yourself. As it seems you don't really want to, just limit yourself to the title:
Syria: UN Inquiry Should Investigate Houla Killings. Where does it say Assad did it?
You don't seem to have read it very attentively then:
According to survivors that Human Rights Watch interviewed and local activists, the Syrian army shelled the area on May 25, and armed men, dressed in military clothes, attacked homes on the outskirts of town and executed entire families.
All of the witnesses stated the armed men were pro-government, but they did not know whether they were members of the Syrian army or a pro-government militia, locally referred to as shabeeha.
.....
“There’s no way a Syrian military commission can credibly investigate this horrendous crime when so much evidence suggests pro-government forces were responsible,” said Sarah Leah Whitson, Middle East and North Africa director at Human Rights Watch. “Annan should insist that Syria grant access to the UN commission of inquiry to investigate this and other grave crimes.”
Perhaps this might assist?
GENEVA (AP) — The U.N.'s top human rights body harshly condemned Syria on Friday for the massacre last week of more than 100 civilians, apparently at the hands of government troops and pro-regime thugs.
....
According to preliminary U.N. investigations, at least 49 children under the age of 10 were among the dead — with entire families apparently executed in their homes. U.N. investigators have said there is strong evidence that pro-regime fighters were behind the massacre.
AND THIS:
Syrian government forces and militia loyal to the Assad regime are killing and sexually abusing children and using them as human shields, the UN says, amid fears that the conflict is intensifying.
The UN secretary general Ban Ki-moon's annual report on children and armed conflict during 2011 included Syrian government forces and the allied shabiha for the first time on a list of 52 governments and armed groups that recruit, kill or sexually attack children in armed conflicts.
"In almost all recorded cases, children were among the victims of military operations by government forces, including the Syrian armed forces, the intelligence forces and the shabiha militia, in their ongoing conflict with the opposition, including the Free Syrian Army," the report says.
http://www.guardian.co.uk/world/2012/jun/12/syrian-forces-killing-children-un
UN and HRW seem to be making their position quite clear, based on eye witness interviews.