The transgenic calf cloned this summer at the Estonian University of Life Sciences died unexpectedly late Friday night.
Scientists say a macropathology report indicated no cause of death and they will have delve deeper, the university said on Sunday.
"The death of the cloned calf was very unexpected as the critical period in the calf's development had passed and its health was good. Today veterinarians who have been constantly observing the calf can't say what might have caused the sudden death, but we will conduct all the possible tests to determine the cause," said Ülle Jaakma, dean of research.
Jaakma said the process would take several weeks.
Juuni (June), the calf, had a human growth hormone sequence in its genome and was expected to yield milk that could be used to treat a number of disorders.
Juuni was only the second such calf in the world.