No drugs were found in his system, the sheriff said.
The investigation is ongoing, with criminal charges still possible, Irvine Police Lt. Mark Anderson said.
"We remain shocked and saddened by Noah's tragic death, and we offer our deepest sympathies to the Domingo family for their loss," according to a UC Irvine statement. "His death brings an urgent focus on alcohol and substance abuse, from the cultural pressures that encourage unhealthy behavior to the policies designed to mitigate danger."
The school suspended Domingo's fraternity, Sigma Alpha Epsilon, shortly after his death and the house remains closed.
Domingo was from La Crescenta, about 60 miles northwest of Irvine.
David K. Li
David K. Li is a breaking news reporter for NBC News.