The Natural Resources Defense Council's Chuck Clusen reiterated the concern, telling the Associated Press, "This string of mishaps by Shell makes it crystal clear that we are not ready to drill in the Arctic. Shell is not Arctic-ready. We have lost all faith in Shell, and they certainly don't have any credibility left."

The Noble Corporation, which operates two fleets for Shell in the Arctic Ocean, issued a written statement saying the company is conducting an internal investigation, which has uncovered "other potential regulatory non-compliance issues that it discovered relating to its operations, including possible unauthorized collected water discharges outside the period of drilling operations."

The statement added, "Noble continues to cooperate with the Coast Guard's ongoing review... The Company cannot make any assurance as to the timing, impact or outcome of the Coast Guard investigation or its own investigation into these regulatory matters."

On Thursday, a coalition of 46 Democratic members of Congress called on Secretary of the Interior Ken Salazar to join with the Coast Guard in investigating the whether the fleet contracted by Shell should operate in the Arctic Ocean.

Sources tell CBS News that Coast Guard Investigations Service agents and EPA investigators have been meeting with federal prosecutors in Anchorage. Electronic information downloaded from the Noble Discoverer's on-board computers is being reviewed by Coast Guard experts in Washington.

A spokesman for U.S. Attorney Karen Loeffler did not return a call seeking comment. A Coast Guard spokesman when asked whether the criminal probe might result in charges would only say, "the Coast Guard continues to review evidence and the investigation is ongoing".