According to police, Renn and another officer responded to a call about shots being fired in the 3400 block of Forest Manor, went to the alley and approached a group of unidentified subjects. As officers approached, 25-year-old Major Davis Jr. of Indianapolis began shooting, prompting officers to return fire, police said.
Davis also was struck and remains in critical condition following surgery. Police said he is preliminarily charged with murder.
An investigation is ongoing.
"This is a sad day for the IMPD family and the community as a whole," said IMPD Chief Rick Hite. "Please keep Officer Renn's family in your prayers. We will remain vigilant as we continue to take care of one another and the city in the days and weeks to come."
Public Safety Director Troy Riggs said Renn was the eighth police officer shot during the last 18 months.
The shootings are the latest in a violent year for Indianapolis, which is on track to rival its record of 162 homicides, set in 1998. A shooting in a bar-hopping Indianapolis neighborhood injured seven people early Saturday.
"Our officers less than 24 hours ago were attending to the wounded citizens of this city," Hite said, while standing alongside Riggs and Mayor Greg Ballard outside the hospital Saturday night. "What are we going to do about people in the community who . welcome us with assault weapons when we are sending out officers who are trying to protect them?"
Ballard said it was "a time to grieve and we want to send our hearts out for the family of Officer Renn."
Fraternal Order of Police Vice President Rick Snyder said Renn was "heroic" his entire career.
"For 20 years he did his job for the citizens and for the city," Snyder told The Indianapolis Star (http://indy.st/1qZePjg). "He put on a badge and went to work knowing the risks. Unfortunately, he is not going home tonight and a family is ruined."