At a press conference Friday in Winnipeg, she called on the federal government to take action to address why aboriginal women are so vulnerable.
Ashton was joined by Manitoba First Nations leaders, who have also made a public plea for a national inquiry.
First Nations leaders made the call on Tuesday evening, on the steps of the Manitoba legislature, where 300 people had marched, demanding answers to the problem of missing and murdered aboriginal women.
They said recent events in Winnipeg show the government's current approach isn't working.
Shawn Lamb, 52, was charged Monday with three counts of second-degree murder in connection with the deaths of Tanya Nepinak, 31, Carolyn Sinclair, 25, and Lorna Blacksmith, 18.
All three aboriginal women went missing in the past year.
Sinclair's body was found in a dumpster behind an apartment complex on Notre Dame Avenue in March, while Blacksmith's body was found in a yard on Simcoe Street late last week.
The body of Nepinak, who went missing last September, has yet to be found but police believe she is dead.
Her family is scouring the city's riverbanks, looking for signs of her.
Ashton and the leaders have drafted an open letter to the Prime Minister requesting a response by July 10.
The federal government has already rejected calls for an inquiry.
But Ashton said the government cannot ignore what she calls an epidemic.
"We can no longer say this isn't happening. We must listen to the voices of aboriginal women and aboriginal communities. I mean, how many families are now torn apart by this tragedy?" she said.