The International Port of Coos Bay published a document on their website Monday announcing the departure of Mitsui & Co, the U.S. subsidiary of a Japanese trading company, and Korean Electric Power Corp., a potential coal buyer, from the project.

Californian Metropolitan Stevedore Company, known as Metro Ports, is left to decide whether to continue with the plan to ship coal from mines in Montana and Wyoming to Asia.

Metro Ports signed a renewal of its exclusive negotiating agreement with the port on March 5, giving it until the end of the month to decide whether to continue.

It is unclear why Mitsui and KEPCO have withdrawn but Sierra Club spokeswoman Krista Collard told the Associated Press that it may be due to high infrastructure costs.

“They are going to have a lot of difficulty moving forward, because of the infrastructure upgrades necessary both to the rail lines and the bridges and overpasses they are asking the partner investors to foot the bill on,” she said.

Coos Bay, one of five northwest ports interested in exporting coal to Asia, has received serious opposition from environmental groups who argue that burning coal in Asia is bad for global warming, and that coal dust from trains traveling to the port can have adverse health effects on local communities.

A 2012 feasibility study for the project estimated that exports through Coos Bay could amount to 10 million metric tons by its fifth year in operation.

Other ports pursuing coal exports are Cherry Point and Longview in Washington, and St. Helens and Morrow in Oregon.