Two recent oil pipeline spills have prompted new criticism from opponents of the proposed Keystone XL project, while raising more questions about whether the federal government is adequately monitoring the nation’s vast labyrinth of pipelines.

An Exxon Mobil pipeline ruptured in central Arkansas on Friday, leaving a sheen of oil on nearby streets and causing the evacuation of 22 homes in the small town of Mayflower.

Exxon Mobil said its Pegasus Line, which runs from Patoka, Ill., to Nederland, Tex., was carrying heavy crude from western Canada when the spill occurred. On Tuesday, Attorney General Dustin McDaniel of Arkansas announced that he was opening an investigation into the spill, and he asked Exxon to preserve all documents related to the accident.

Opponents of the Keystone XL pipeline, which would also move heavy Canadian crude, leapt on the Exxon spill, reiterating their contention that crude drawn from Canada’s tar sands region is too risky to transport and especially vexing to clean up.