Safe weight loss = 1-2 pounds per week? Who says?

I’ve spent some time looking for the original research that supports the common advice I hear from medical professionals that “safe” weight loss is 1–2 pounds per week. In terms of the math involving calories in a pound and exercise, that 1–2 pound number seems reasonable for someone who is moderately overweight, but I’d like to see the research and know who was studied—if there was, in fact, a study? Was it overweight people, obese people, or morbidly obese people? Who performed the study? What were the parameters? And what is the term “safe” implying in this context?
It does seem to me that morbidly obese people could “safely” lose weight at a rate of 10–15 pounds/month, but without knowledge of what is meant by safe and whether the 1–2 pound number is universal or not it’s hard to know.
Anyone know where the study/ies is/are?