An arrow tore through an artery which caused the Ice Man to have a heart attack and go into shock.
The report was published online in the Journal of Archaeological Science. Oetzi was found in the Swiss Alps in 1991 10,500 feet above sea level between the borders of Austria and Italy.
Oetzi was either a hunter or a warrior from the Neolithic Age. He was found with a bow, a quiver of arrows and a copper ax. He or someone he was with had removed the arrow that caused his death. That would have hastened his death.
The mummified body is being preserved in Bolzano, Italy. Researchers have studied Oetzi to gain more knowledge of the people who lived between 3,300 to 3,100 B.C.
Earlier speculations of the mummy's death were ritual sacrifice or being lost in the elements. The fact that the arrowhead was embedded in the body makes for the likelihood that the man was in action at the time of his death.
"They've applied noninvasive techniques from medical imaging to a specific question and have confirmed that it was the arrow which killed Oetzi, without having to thaw him out," Dean Falk, professor of anthropology at Florida State University, said in a telephone interview.
"I think it's very illustrative of the importance of these new techniques to science," said Falk, who had previously studied the corpse but did not take part in the latest research.
Had Oetzi suffered the wound today he would still only have a 40% survival chance.