The long-awaited and problem-plagued inquiry into murdered and missing Indigenous women and girls will forge ahead as planned following the sudden resignation of one of its commissioners, says Indigenous and Northern Affairs Minister Carolyn Bennett.

Marilyn Poitras stepped down from the five-member commission, citing issues with the "current structure" of the inquiry.

In a letter of resignation obtained by CBC News, Poitras said she still supports the principles of the inquiry.

"It is clear to me that I am unable to perform my duties as a commissioner with the process designed in its current structure," she said in the letter to Prime Minister Justin Trudeau.

In a two-page statement released this afternoon, Poitras, who is Métis, said she realized the vision she held was shared by few within the national inquiry, and the "status quo colonial model" of hearings is the path for most.

"Because of this, I strongly feel the terms of reference that we were set out to achieve have not been met," she wrote. "This is why it is with great regret and a heavy heart that I resign my position as commissioner, effective July 15."

Poitras said the incidence of violence and suicide is not the whole picture of who Indigenous people are, and that there is a "whole resiliency piece" that no one is talking about.

READ MORE: Full Poitras statement

"I believe part of the solution is to draw on our strengths and resiliency as Indigenous people, because if all we ever talk about is 'the Indian problem,' then we'll only ever be 'the Indian problem,'" she said.

Four other staff members of the inquiry have already left since June.

Bennett said the government will decide whether to replace Poitras after consulting with the independent commission, but stressed that it is important to have a strong Métis "lens" on the commission. The four other commissioners were more than prepared to move forward and committed to doing the work, she said.

Indigenous Affairs Minister Carolyn Bennett speaks to reporters in the Commons Foyer 2:01

The resignation is the latest in a series of setbacks, delays and criticisms.

Asked if the inquiry is doomed to fail, Bennett said the commission remains firmly committed to the task and that many families see this as the "chance of a lifetime" after waiting many years.

"A lot of the families are saying to me, calling me, and saying 'This has to continue, we've waited 30 years for this. Please let the commission continue its work,'" she said.

"We are very aware of the concerns that have been expressed but we are also listening to families."

Better communications required

Bennett said she spoke with the commissioners in a "very productive" meeting Monday. She said she is confident they have the vision, values, tools and plan to get the work done, but said they must do a better job of communicating their task.

"Successful commissions bring Canadians with them as they go," she said.

'We want the commission to be able to explain their plan,' says the Minister of Indigenous Affairs. 7:11

Chief commissioner Marion Buller declined to speculate on Poitras's departure, but conceded that "like any family, there's not always harmony."

"I think it's fair to say that when you have five people in a room, you're going to have five different opinions," she told CBC News Network's Power & Politics host Rosemary Barton.

Resignation 'regrettable'

Assembly of First Nations National Chief Perry Bellegarde issued a statement calling the resignation "regrettable."

"Our first thoughts are for the families of missing and murdered Indigenous women and girls and how this affects them and their path to healing and justice," he said. "We're very concerned about this resignation because the work of the national inquiry is too important and we want to see it succeed for the families."

Poitras served as an assistant professor at the University of Saskatchewan's College of Law.

She began her career as a Native court worker and eventually received a masters of law degree from Harvard University. She has focused on constitutional and aboriginal law, and has placed a special focus on legal education initiatives.

Sources tell CBC News there was some friction between Poitras and the other commissioners and that there was a difference of opinion on the way in which the inquiry should be conducted.

The Ontario Native Women's Association issued a statement withdrawing support for the inquiry in its current format and approach.

President Dawn Harvard said the organization's members still want an inquiry, but "no longer have faith that this inquiry will meet its mandate and work responsibly with families and communities."

The association cited a number of concerns, including what it sees as a lack of culture and ceremony, a lack of commitment to reaching out to families, poor communications and a focus on legal matters instead of on prevention.

"We want the inquiry to provide families with the opportunity to be able to share the stories of their loved ones. We want families to get the answers that they need and for their experience at the inquiry to be part of a larger healing journey," Harvard said in the statement. "We also want the inquiry to honour the women and girls, to hear the lessons from their stories, and transform these lessons into powerful recommendations for governments, institutions and communities to take up."

Staffers step down

Last week the inquiry announced plans to conduct hearings across the country later this fall beginning in Thunder Bay, Ont.

Buller spoke today with the CBC's Rosemary Barton on Power and Politics 0:58

At that time, Buller was asked about the resignation of staffers from the inquiry, including executive director Michele Moreau.

She defended their decisions to leave, saying some left for "opportunities of a lifetime" and that turnover is expected.

In a statement released today, Buller thanked Poitras for her contributions to date, and said that "now, more than ever," the commission remains focused on the important work.

"This is a challenging mission and we owe it to you, families, survivors and families of the heart to meet every challenge head on and to persevere for the spirits of those who have been lost and stolen," she said. "Together, the commissioners and staff of the national inquiry will get through this and we thank you for your ongoing support and advice. We are listening."

Buller said the inquiry will forge ahead with community visits this summer in preparation for fall community hearings.