An estimated 670 million litres of waste water gushed out of a broken earth berm at the Obed Mountain mine near Hinton on Oct. 31, 2013.
Peter Murchland, a spokesman for the regulator, says investigators have almost completed the second part of a three-phase probe into what happened.
He says the agency is working closely with Environment Canada on the investigation, but could take enforcement action on its own.
The mine was owned at the time by Sherritt International, but has since been sold to Westmoreland Coal Co.
Conservation groups are questioning why no charges have been laid and say more details about the spill and how it affected aquatic life, the river and people who live downstream should be made public.
"Upon completion of the investigation, an investigation summary report will be produced for review by AER leadership to determine if an enforcement action is required; “no action” is also a potential outcome," Murchland wrote in an email Friday.
"If an enforcement action is warranted, they can include (but are not limited to) a warning letter, administrative penalties (fines), prosecutions and suspension/cancellation of an authorization."
The regulator said material in the spill included minerals, bits of coal and a substance used in coal production.
Last year, an Environment Canada database said the spill contained damaging compounds such as arsenic, mercury, cadmium, lead and manganese.
In the weeks following the spill, the province advised downstream communities not to draw water from the river and farmers not to let livestock drink from it.
Groups including the Alberta Wilderness Association, the Aboriginal Alliance of Alberta and the Athabasca Watershed Society have called on the federal government to charge Sherritt under the federal Fisheries Act.
Hilary Prince, a spokeswoman for Fisheries and Oceans Canada, said department staff are working with Environment Canada to investigate an alleged violation of the Fisheries Act.
She said it is too soon to say if charges will be laid.
"Fisheries and Oceans staff have conducted numerous on-site visits, collected a wide range of evidence and served legal documents to support and further this investigation," she wrote in an email Friday.
"These types of investigations are complex and normally require considerable time to complete before charges might be laid."
Bruce Maclean of the Mikisew Cree and Athabasca Chipewyan First Nations Community Monitoring Program said the tailings pond structure at the Obed mine had passed inspection only weeks before it failed.
The conservation groups say a tailings dam breach Aug. 4 at the Mount Polley gold and copper mine in British Columbia has also raised questions about the inspection of such containment ponds.