U.S. regulators were investigating BP Plc on Monday for possible insider trading related to its Gulf of Mexico oil spill, a move that may hurt the energy giant's efforts to restore investor confidence.

Details of the probe emerged as BP prepared to deliver the first of what it hopes will be two knockout blows to "kill" its ruptured Macondo well, 105 days after it started gushing out millions of gallons of oil, causing an environmental disaster.

Two sources familiar with the preliminary Securities and Exchange Commission probe said the alleged insider trading took place after the start of the BP oil spill on April 20.