They were the first police officers to lose their lives in a terrorist incident in the city for 50 years.
The ambush 40 years ago came just three days before Bloody Sunday sent shockwaves right across the Province.
But now a former RUC officer, who was in the car when the officers were killed, has revealed Northern Ireland's Deputy First Minister, Martin McGuinness is under the spotlight for his alleged involvement.
The retired officer, who did not want to be named, said he was told by a member of the Historic Enquiries Team investigating the case that McGuinness is the main suspect.
"A representative of the HET called to interview me in relation to the attack where two of my colleagues died and he told me Martin McGuinness was thought to have been one of the IRA men who ambushed us," he said.
The former constable was with his young colleagues in an unmarked patrol car as they travelled to Rosemount Station to end their shift.
The 59-year-old described how the driver of the patrol car tried to 'zig zag' out of danger as gunmen opened fire on them as they travelled along Creggan Road and Helen Street.
Sergeant Gilgunn and Constable Montgomery were shot dead but the driver managed to managed to get the car on down the road to Rosemount and stopped at the station.
The officer said that after removing a badly wounded colleague from the car, he went back for Constable Montgomery.
"When I got to the car and saw him I knew he was gone.
"He had actually been shot in the back. The rounds had gone through the boot of the car and then the seat before he was hit.
"I had swapped places with Davy in the back of the car about 20 minutes before the attack... it just wasn't my time to go," he said philosophically.
When asked by the Herald what happened in the aftermath of the murders he replied simply "not much".
"I was in the station and one of the bosses spoke to me for a couple of minutes and after an hour or so I was told to go on home. It was never mentioned to me again. I took a few weeks off and was transferred to another county. That was it.
"I sometimes have a bemused think to myself nowadays when I read of people complaining that various investigations could have been more thorough. No one even took a statement from me, but I was just a cub and assumed others knew what they were doing.
"The HET people contacted me and I agreed to meet with them. I wanted to see what I could find out from them as much as anything else. After we discussed the actual shooting they wanted to know if any suspects' names were being bandied about at the time. I told them I never heard, I wasn't there after it," said the retired policeman.
"They asked who I thought was involved and I said you tell me. When pushed on this, one of them said, 'The Deputy First Minister, McGuinness himself'."
Sgt Gilgunn was married and had an eight-month-old baby. Constable Montgomery was planning his engagement party just hours before the attack.
McGuinnness's involvement has been speculated about before in relation to these murders.
As a result of testimony given to the Saville Inquiry, set up to investigate events on Bloody Sunday, it was stated that McGuinness was second in command of the IRA at the time and was seen with a sub machine gun that day.
A sub-machine gun was used in the murders of Sgt Gilgunn and Constable Montgomery.
This latest revelation comes after Mr McGuinness was challenged by the son of an Irish Army Private who was killed by the IRA during the Don Tidey kidnapping episode in 1983.
A spokesperson for the Historic Enquiries Team said: "The HET deals with bereaved families on a strictly confidential basis. It does not discuss the content or progress of reviews with anyone except the family concerned."
Recently presidential candidate McGuinness claimed that while he fired a gun during the Troubles, he did not kill anyone.
He said: "I didn't say I never fired a gun -- I was in the IRA. There were battles on the streets of Derry. I've never run away from that."
When asked if he had killed anyone, he answered no.
During his campaign earlier this month, McGuinness was confronted by David Kelly, the son of Private Paddy Kelly who was killed alongside Recruit Garda Gary Sheehan by the IRA in Leitrim, November 1983. McGuinness was canvassing in Athlone at the time when confronted by Mr Kelly who urged him to name his father's killers.
- Sean won't return to Dragons' Den
- More car trouble for Dana with parking ticket
- I'm no party hack says mum who challenged Gallagher