TORONTO -- TransCanada Corp. has cancelled its $15.7-billion proposed Energy East pipeline, with CEO Russ Girling citing non-specific "changed circumstances" Thursday morning. Supporters of the Calgary-based company's project -- which would build and operate a 4,500-kilometre pipeline from Alberta to the East Coast -- are casting the blame on costs and delays from federal government regulatory meddling, while other point to simple market forces. Here's a timeline of events in the controversial pipeline's history.

Aug. 1, 2013: TransCanada announces at the height of the oil boom that it is officially moving ahead with the Energy East pipeline project, promising jobs, tax revenue, and energy security.

Oct. 29, 2014: A report released by environmental groups questions whether Energy East is necessary to supplant Eastern Canada's oil imports from the foreign suppliers frequently mentioned by TransCanada. The company has said repeatedly that eastern Canadian refiners rely on imports for 86 per cent of their daily needs.

May 13, 2015: More than 60 organizations call on the National Energy Board to suspend TransCanada's application for the Energy East Pipeline. Groups including Greenpeace Canada, the Council of Canadians and the David Suzuki Foundation said in a letter to the federal pipeline regulator that it should suspend the review process while waiting for new information from the company.

July 31, 2015: TransCanada warns the $12-billion price tag for its Energy East pipeline project is expected to grow. The company says a decision in April to relocate a proposed marine terminal at Cacouna, Que., due in part to concern for beluga whales near the site, will increase costs.

Aug. 19, 2015: TransCanada says concerns raised about Energy East in a report about the potential impact of the project on whales and some fisheries in the Bay of Fundy are unfounded. A report released by the Conservation Council of New Brunswick said noise from tanker traffic causes heightened levels of stress for the North Atlantic right whale, the most endangered large whale in the world.

Oct. 8, 2015: Environmental group Environmental Defence says the National Energy Board is rushing the process for Energy East by gathering oral traditional evidence from aboriginal bands before it has received a complete application.

Dec. 17, 2015: TransCanada files an amended application and cost estimate of $15.7 billion for Energy East. The company says the filing with the National Energy Board makes nearly 700 changes to the route in response to concerns for environmentally sensitive areas.

Feb. 3, 2016: The National Energy Board directing TransCanada to rework its application to build the contentious Energy East pipeline because the document is too hard to understand -- even for experts.

March 1, 2016: National cohesion on Energy East drifts further out of reach after the Quebec government announced plans to seek an injunction against TransCanada. Environment Minister David Heurtel said his government got no response from the company to two letters it sent in late 2014 advising that the project must pass a provincial environmental impact assessment.

March 7, 2016: Protesters in Quebec disrupt hearings into the Energy East project seconds after they officially started. The province's environmental regulation agency quickly suspended hearings into the project, which were meant to conduct an environmental assessment of the risks and benefits and produce a report to Quebec's environment minister.

June 16, 2016: The National Energy Board says it has officially started the clock on its review of Energy East. The NEB now has 21 months to hold hearings along the 4,500-kilometre route from Alberta to New Brunswick and report to government whether the pipeline is in the national interest.

Aug. 30, 2016: All future National Energy Board hearings into TransCanada's Energy East pipeline project are suspended until the board rules on motions demanding two of three panel members resign. The NEB board received the motions after a news report revealed the two members met with ex-Quebec premier Jean Charest, who was at the time a paid consultant for TransCanada.

Sept. 9, 2016: The National Energy Board sidelines all three Energy East reviewers following complaints that two of them met privately with a TransCanada consultant the year before and discussed the proposed oil pipeline.

Jan. 27, 2017: The new National Energy Board panel tasked with reviewing the Energy East pipeline decides to throw out all of the decisions made by the previous panel. The board said all hearing steps and related deadlines for the TransCanada Corp. project are no longer applicable as the new panel begins its review.

Aug. 23, 2017: In a decision cheered by environmentalists but considered a setback by the oil industry, the National Energy Board says it will allow wider discussion of greenhouse gas emission issues in upcoming hearings for the Energy East Pipeline. It says it will for the first time consider the public interest impact of upstream and downstream GHG emissions from potential increased production and consumption of oil resulting from the project.

Oct. 5, 2017: TransCanada announces it is no longer going ahead with the Energy East pipeline project.