(CNN) -- Louisiana Attorney General Charles Foti Jr. has issued 73 subpoenas in an investigation into allegations that euthanasia may have taken place at one of the hospitals flooded by Hurricane Katrina, he told CNN Wednesday night.The subpoenas were served on employees of all levels at Memorial Medical Center, which is owned by Tenet Healthcare, because "cooperation, lately, has not been as good as I had hoped," Foti said. The subpoenas require that people appear before investigators for questioning."Some people were not coming forward. We learned Tenet sent out a letter that had a chilling effect," Foti said. "We had no choice but to issue these subpoenas.""They [Tenet] seem to be in a position of protecting themselves, while we are just trying to get to the facts of what happened at the hospital," the attorney general said.CNN obtained the memo -- dated October 14 -- to which Foti was referring. In it, Tenet's assistant general counsel, Audrey Andrews, advised staff members that "in the aftermath of Hurricane Katrina, you may be contacted by a government representative or a representative of the media.""In fairness to you, if you are contacted by a representative of a state or federal agency, or if you are contacted by the media, you may wish to first confer with legal counsel. You have certain legal rights about which you should be aware."First, you have the right to decide whether or not you wish to be interviewed. You can consent or decline. The decision as to whether or not you consent to be interviewed is yours alone," Andrews said.However, she added that if anyone spoke to investigators, they were obliged to "provide truthful information in response to questioning."Three days after Katrina flooded most of New Orleans, staff members at Memorial had repeated discussions about euthanizing patients they thought might not survive the ordeal, according to a doctor and nurse manager who were in the hospital at the time. Katrina came ashore August 29. (Full story)After allegations of mercy killings surfaced, Foti's office asked that autopsies be performed on all 45 bodies taken from the hospital after the storm.Tenet has told CNN that most of the 45 patients who died were critically ill, and about 11 patients died the weekend before the hurricane struck and were placed in the morgue."We have asked for certain records from Tenet 15 days ago, and we have yet to receive them," Foti said. "We have also asked for the location and address of every employee working at the hospital at the time of the hurricane, and they have not provided that either."A spokesman for Memorial denied that the hospital has been uncooperative."We have never discouraged any employee from working with the Louisiana attorney general's office. In fact, we know that some have already spoken to his representatives. We've been cooperating with the attorney general's office, and have spoken regularly about employee interviews," the spokesman said.