NEW ORLEANS - The federal government announced criminal and civil investigations into the Gulf of Mexico oil spill Tuesday, sending BP's stock plummeting and taking much of the market down with it.

BP engineers, meanwhile, tried to recover from a failed attempt to stop the gusher with an effort that initially will make the leak worse.

Attorney General Eric Holder, visiting the Gulf to survey the fragile coastline and meet with state and federal prosecutors, would not say who might be targeted in the probes into the largest oil spill in U.S. history.

"We will closely examine the actions of those involved in the spill. If we find evidence of illegal behavior, we will be extremely forceful in our response," Holder said in New Orleans.

BP's stock nose-dived, losing nearly 15 percent of its value on the first trading day since the previous best option, the so-called "top kill," failed and was aborted at the government's direction. It dipped steeply with Holder's late-afternoon announcement. Other energy stocks tumbled with it, causing the Dow Jones industrial average to fall 112.

After six weeks of failures to block the well or divert the oil, BP was using robotic machines to carve into the twisted appendages of the crippled well. The latest attempt involved using tools resembling an oversized deli slicer and garden shears to break away the broken riser pipe so engineers can then position a cap over the well's opening.

Even if it succeeds, it will temporarily increase the flow of an already massive leak by 20 percent, at least 100,000 gallons more a day. And it is far from certain that BP will be able to cap a well that one expert compared to an out-of-control fire hydrant.

"It is an engineer's nightmare," said Ed Overton, a Louisiana State University professor of environmental sciences. "They're trying to fit a 21-inch cap over a 20-inch pipe a mile away. That's just horrendously hard to do."

"If all goes as planned, within about 24 hours we could have this contained," BP's Doug Suttles said yesterday after touring a temporary housing facility set up for cleanup workers in Grand Isle. "But we can't guarantee success."

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