Pop superstar Whitney Houston was seen ordering and consuming considerable quantities of alcohol at The Beverly Hilton two mornings last week, said a source briefed on her behavior and activity in the days before her death Saturday.

The source told CNN's Don Lemon that Houston ordered the drinks before 10 a.m. last Wednesday and Thursday from the bars in the lobby and pool area.

Guests both days overheard Houston loudly complaining about her drinks, accusing bartenders of "watering down" or "putting too much ice" in them, the source said Wednesday.

The source said other guests expressed concern about Houston's erratic behavior. Her disheveled appearance, including mismatched clothing, suggested to them that she was intoxicated, the source said. The source said Houston was seen jumping in and out of the pool and doing somersaults in the pool area.

The source added that Houston was alone for some of the time, but was also seen at the pool drinking with her entourage and a male companion on both days.

Saturday, the day she died, Houston was seen drinking at the pool in the morning, although the source noted witnesses said her behavior did not appear erratic.

A singer who participated in an impromptu duet with Houston at a party Thursday night said Houston was not behaving erratically, but did have champagne.

"I didn't see someone who was high," Kelly Price told CNN's "Starting Point" on Monday. During the party, Houston took the stage unexpectedly and sang a hoarse rendition of "Jesus Loves Me" with Price.

Authorities are trying to determine the source of Houston's prescription medication found in the hotel room where she died, officials said Wednesday.

The Los Angeles County Coroner's office has issued subpoenas seeking Houston's medical records and her prescriptions, Ed Winter, assistant chief coroner, said Wednesday. Investigators are also contacting pharmacies where the prescriptions were filled, he said.

Houston, 48, was found dead in the bathtub of her suite at the Beverly Hilton on Saturday, hours before she was to attend a pre-Grammys bash at the Beverly Hills, California, hotel.

Houston's death certificate, filed Wednesday, listed her cause of death as "deferred," meaning it was delayed pending more information. Speculation has grown while authorities await the outcome of toxicology tests that could take weeks.

Winter has said that while medication was found in the room, the amount was less than that usually present in overdose deaths.

"I know there are reports that she maybe was drowned or did she overdose, but we won't make a final determination until all the tests are in," Winter said earlier. He ruled out foul play and said there were no injuries to Houston's body.

The prescriptions found in the room were in Houston's name, Winter told Lemon. By contacting the doctors, investigators are attempting to verify the prescriptions and find out if there were any more.

Several doctors have been contacted in California and other states, along with pharmacies, he said. All were cooperating.

"The doctors I've contacted so far have been helpful," he said, and nothing out of the ordinary was found as long as Houston was taking the medication as prescribed. Some additional information has been gained, he said, and investigators are waiting to see Houston's medical records.

It does not appear that she was "doctor shopping," he said, but officials are attempting to contact as many doctors as possible to rule that out, along with determining whether Houston was filling multiple prescriptions.

"Everything is above board," Winter said. "So far, nothing looks criminal."

But it was too soon to say whether Houston had any prescriptions she should not have or used an alias, he said.

Although subpoenas have been issued -- a fairly unusual step in a death investigation -- Winter said the federal Drug Enforcement Administration has not been asked to assist.

As part of the investigation following the death of singer Michael Jackson in 2009, bags and bottles of prescription drugs were found at Jackson's home. Local and state authorities spent months tracking the pharmacies that filled the prescriptions. One of those pharmacies was the Mickey Fine Pharmacy and Grill in Beverly Hills.

One of the prescriptions found in Houston's suite was also from the Mickey Fine pharmacy, Winter said, but "the prescription that came from Mickey Fine is not something that would kill her."

Investigators want a history of all prescriptions Houston may have filled there and at other pharmacies, and wants to make sure all of the pharmacies that filled Houston's prescriptions are accounted for. Generally, "you go to a doctor and get a prescription, then you go to the dentist and get another prescription," he said.

Houston visited a Beverly Hills ear, nose and throat specialist four days before she died, a source familiar with her visit said Wednesday.