The U.S. government has launched a widely expected criminal and civil investigation into BP’s massive oil spill in the Gulf of Mexico, U.S. Attorney General Eric Holder said on Tuesday, ratcheting up pressure on the beleaguered British oil company.

"We have begun both a criminal as well as a civil investigation as is our obligation under the law," Holder told reporters after meeting with state and federal prosecutors in New Orleans. "Our environmental laws are very clear."

Federal agencies, including the FBI, are participating in the probe and "if we find evidence of illegal behavior, we will be forceful in our response," he said, adding that prosecutors had a "sufficient basis" to start a criminal probe.

During his first visit to the disaster site where he took a a tour of the spill, Holder said he saw "oil for miles and miles, oil that we know has already affected plant and animal life along the coast."

Confirmation of the probe, which Holder said started "some weeks ago," sent BP's shares down further after already getting hammered most of Tuesday. The stock plunged 15%, or $6.43, to close at $36.52 at the end regular trading on the New York Stock Exchange.

Pressure Growing

Pressure has been building on BP and the Obama administration to stop the spewing oil, now in its 43rd day and no end in sight. Legal experts had predicted that it was only a matter of time before a criminal and civil probe would begin.

Holder declined to say who were the targets of the investigation, but the Justice Department has already demanded that at least three companies involved in the spill, BP, Transocean and Halliburton preserve their records related to the accident.

The Justice Department will look for violations of the Clean Water Act, the Endangered Species Act, the Migratory Bird Treaty Act and the Oil Pollution Act of 1990, which can be used to hold them liable for cleanup costs and reimbursement for government efforts. Additionally, Holder said that "nothing is off the table at this point" with regard to the range of charges prosecutors could pursue, including traditional criminal charges if they find false statements were made.

"As our review expands in the days ahead, we will be meticulous, we will be comprehensive, and we will be aggressive," Holder told reporters. "We will not rest until justice is done."

However, he did acknowledge that the government's first priority was to stop the gushing well and clean up the oil. The April 20 explosion aboard the Transocean Deepwater Horizon rig killed 11 workers and injured 17 others. Holder said that there could also be charges related to the "untimely and tragic deaths of those 11 rig workers."

This is not the first time BP has been in the Justice Department's sights. In October 2007 the company agreed to pay more than $370 million in fines and restitution to resolve several environmental and fraud allegations including criminal charges. Those included a deadly explosion at a Texas refinery in 2005, pipeline leaks at the Prudhoe Bay field in Alaska in 2006, an an attempt to corner the U.S. propane market in 2004.