BOULDER, Colo. -- Traces of a "date rape" drug were found in blood samples from two of the nine women who were hospitalized after attending University of Colorado fraternity parties last month, police said Thursday. One of the doses was potentially fatal.

"This is a frightening situation, to think that someone is spiking the drinks of women with a dangerous drug at these parties," Police Chief Mark Beckner said in a statement.

The parties were held Sept. 24, and police said most of the women taken to the hospital had been drinking at either the Sigma Pi or Phi Kappa Tau fraternities, which are off campus

Blood tests found evidence of gamma hydroxybutyrate (GHB), considered a date rape drug, in two of the women.

"Of these two samples, one tested at a level identified as having been a fatal level in the past," police said.

Police responded to the first call of a sick woman at a sorority house at about 1 a.m. Saturday. Another call came in from the same sorority house a short time later of a woman passed out in a hallway.

Police checked the hospitals and found three other women who were hospitalized after heavy drinking.

Two additional sorority members -- one found at her apartment and another who was transported to the hospital -- were also found by Boulder police.

Campus police were also investigating two additional cases, bringing the total to nine.

Brooks said eight of the nine were either 18 or 19 years old. Their blood-alcohol level was not released but Brooks said officers, who have the discretion to take intoxicated people to a detox center or place them in the custody of sober people, felt the women needed to be hospitalized.

On Thursday, CU issued a statement that said it was adding $1,000 to a reward for an arrest and the filing of charges against those responsible:

"We are deeply concerned to learn that some of the women involved in the recent fraternity parties appear to have been drugged. The use of the GHB drug is a reprehensible act and we hope the Boulder Police Department is successful in identifying the perpetrators. To assist in their efforts, CU-Boulder is adding $1,000 to Boulder County Crime Stoppers for information leading to an arrest and filing of charges in this case.

"Based on the results of the police investigation, we also will hold accountable any students found to be responsible for these life-threatening actions. We also expect the fraternities' national offices to take serious and appropriate action.

"This new information underscores the importance of actions by individuals who seek medical attention for those affected. In that regard, our Good Samaritan policy allows us to take into consideration the provision of emergency medical services as a mitigating factor in our judicial affairs process.

"We join the Boulder Police Department in urging everyone to be extremely careful when attending social functions involving alcohol."