The National Institutes of Health (NIH) announced Wednesday that it is ending its support for invasive research on chimpanzees and retiring the last of its 50 chimps set aside for future research, bringing to a close years of controversy over federally funded primate experimentation.

NIH Director Francis Collins wrote in an open letter that the chimps, the closest living relatives to humans, are no longer needed to test experimental vaccines, drugs or other treatments. As such, they will be moved into a federal sanctuary - Louisiana's Chimp Haven - as soon as room becomes available, reported the Los Angeles Times.

"It is clear that we've reached a tipping point," Collins said in the statement.

After two and a half years, Collins said no new biomedical research projects requesting chimpanzees have been approved.

"Research with non-human primates has and continues to be vital to helping us understand and improve human health in a multitude of ways, including the development of treatments and interventions. However, use of non-human primates needs to be supported by the science," he said.

"We find no evidence that there is a need to continue to do research of an invasive sort on chimpanzees, not now and not going into the future," he added, according to The New York Times.

Collins cited two events that led to the decision, the first being a 2011 report released by the Institute of Medicine concluding that most research involving chimps was unnecessary for the advancement of the public's health. As a result of the findings, NIH retired 310 of its chimps in June 2013, saying humans' closest relatives "deserve special respect." The remaining 50 chimps were kept in case they were needed for emergency medical research, according to NPR.

The second major development came last summer, when the U.S. Fish and Wildlife Service announced a final rule to classify all chimps, both wild and captive, as endangered.

"I think this is the natural next step of what has been a very thoughtful five-year process of trying to come to terms with the benefits and risks of trying to perform research with these very special animals," Collins said in an interview with Nature. "We reached a point where in that five years the need for research has essentially shrunk to zero. "

While chimp research has ended at NIH, Collins stressed that the agency will continue to conduct medical research on other non-human primates, such as rhesus monkeys, notes the Huffington Post. "These decisions are specific to chimpanzees. Research with other non-human primates will continue to be valued, supported, and conducted by the NIH," he wrote in his statement.

Animal rights groups have been pushing for an end to NIH's chimp research program for years and were quick to praise the decision.

"It's rare to close out a category of animal use so emphatically. That's exactly what's happening here, and it's thrilling," Wayne Pacelle, president and CEO of the Human Society of the United States, wrote in a blog post for the Humane Society.

It's "amazing and historic news," said Kathleen Conlee, vice president of animal research issues for the Humane Society.

However, some animal research advocates were a bit disappointed. "Given NIH's primary mission to protect public health, it seems surprising," Frankie Trull, president of the Foundation for Biomedical Research, told Nature.

Peter Walsh, a disease ecologist at the University of Cambridge, added that the NIH decision "narrows the possibilities for [chimpanzee] conservation research," in part, "because Gabon is the only country other than the United States that allows such work."

"There really is no other place to do conservation-related trials but the U.S. biomed facilities," he says. "A lot of wild chimps died in order to capture infants for originally stocking NIH's own captive populations, and populations they have long supported financially. Now, the first time that NIH has ever been asked to give anything back to wild chimps, they cut and run."