Two vessels collided in the Mississippi River on Wednesday evening, causing a massive oil spill that resulted in a section of the river to be closed for the crew to clean it up, the U.S. Coast Guard revealed on Thursday.

The accident dented one boat that was carrying oil and affected one of its tanks, which caused a large amount of oil to pour into the river.

The Coast Guard said that it's working with the Inland Marine Services, the owner of the boat, and an oil spill response organization to help verity the precise amount of oil that spilled, according to the Associated Press.

The length of the part of the river to be closed remains unclear. The two boats' operators underwent drug and alcohol testing, and results are still pending. They have also been interviewed by Coast Guard agents.

Both vessels were on opposite sides of the river when the collision took place. Lt. Takila Powell said that the crew has begun cleaning up the river and even placed a beam on the cracked cargo tank in order to prevent the leaking of what's left of the oil in the tank.

The crew was able to recover some oil from the river by Thursday, but Powell did not know how much was recovered. She added that since the oil is thick, it has to be heated in order for it to be moved, AP reported

Investigators are currently trying to determine the cause of the collision. The river is currently closed to all traffic, with the exception of boats that have the mile markers 939 to 922, according to The Maritime Executive.