WASHINGTON/HOUSTON - US regulators are investigating insider trading in shares of BP Plc, including whether BP employees profited illegally from 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 spewing millions of gallons of oil, causing an environmental disaster.

The full extent of the disaster was given added clarity on Monday when US government scientists refined estimates of how much oil had flowed into the Gulf of Mexico from the well.