San Bernardino County Sheriff John McMahon identified the deputy Wednesday as Jeremiah MacKay, a man who grew up in the area and followed his father into public service.
MacKay suffered multiple gunshot wounds Tuesday and was transported to Loma Linda Hospital where he died of his injuries. He was 35.
MacKay was a detective who had been with the department for 15 years.
"My condolences go out to the MacKay family," said San Bernardino County Sheriff John McMahon. "This is truly another sad day for law enforcement. Our department is grieving from this event. It's just a terrible deal for all of us."
Another deputy, Alex Collins, was also hit by gunfire and remained hospitalized Wednesday after undergoing multiple surgeries. He was expected to make a full recovery, McMahon said.
MacKay's family is well known in the mountain community, according to locals. His father was a captain in the San Bernardino County Fire Department, and MacKay attended Rim of the World High School.
MacKay spoke to The Associated Press on Saturday while he was helping patrol the Big Bear Lake area for Dorner.
MacKay and his partner had been on duty since 5 a.m. and they were in good spirits. He stood by the door of his car, dressed in full tactical gear with a helmet and bulletproof vest. He took his Mini-14 rifle out and practiced sighting down its barrel, tucking it in against his shoulder and aiming playfully at a snowdrift.
When a reporter walked up behind him, he turned around with a big smile. MacKay said he was happy to be helping the community and that "everyone's been really nice."
It was MacKay's third day helping with the search for Dorner.
As soon as he had heard news about Dorner's pickup truck in the area Thursday morning, MacKay headed out with his partner to help beef up patrol in Angelus Oaks—near the cabin where Dorner made his last stand. MacKay worked into the night and was back the next day at 7 a.m. to help again.
"It's probably one of the biggest tactical and logistics operations we've ever dealt with," MacKay said.
He said it was reminiscent of operations during the 2003 Old Fire, when a blaze in the San Bernardino mountains burned 91,000 acres and torched 1,000 buildings over nine days. But this was entirely different for one major reason.
"This one, you just never know if the guy's going to pop out or where he's going to pop out," MacKay said. "We're hoping this comes to a close without any more casualties. The best thing would be for him to give up."
MacKay said because Dorner was targeting law enforcement, officers were taking extra precautions by staying in pairs and in larger groups.
"Everyone is here for the safety of everyone," MacKay said, "for the safety of each other, for the safety of you."
MacKay leaves behind a wife, a 7-year-old daughter and a 4-month-old son.