(CNN) Nine University of Southern California students have died since the start of the semester, the school said Tuesday -- an unusually high rate that moved university officials to send students a letter about the issue over the weekend.

Three of the nine deaths have been announced since Friday. No foul play is suspected in any of the cases, and suicide is suspected in at least three, school officials have said.

USC President Carol Folt and three other officials sent a letter to students Saturday, partly to counter rumors that suicide was suspected in most of the cases, and partly to urge students to seek help if they need it.

"These student losses are devastating and heartbreaking for all of us," the letter reads. "People are searching for answers and information as we attempt to make sense of these terrible losses."

"There is a great deal of speculation about the causes of these deaths and most are being attributed to suicide. This is not correct," the letter reads. "These tragic losses have resulted from a number of different causes."

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