Political pressures already building on the administration and Congress to do more to deal with the potential devastation facing the gulf and somehow punish BP will likely intensify if the top kill effort fails. And the company would have to try to devise yet another possible solution to a leak that began after an explosion on the Deepwater Horizon oil rig five weeks ago.

After days of delays and testing — and the weighing of several options — BP began shooting as much as 50,000 barrels of heavy drilling fluids into the drill pipes and hoses above the well at 1 p.m. Central time. The procedure has worked around the world to stop leaking wells, but never at the pressures and temperatures found a mile underwater.

Government and BP officials agreed to move forward with the top kill after engineers said their tests showed that the chance of stopping the oil leak was greater than the risk of making it worse.

A 30,000 horsepower engine on a ship floating above the well is pressure-pouring heavy liquids known in the oil business as “drilling mud” or “kill mud” down through drill pipes and hoses attached to the five-story-high blowout preventer sitting on top of the well. Once the liquids have forced the oil back, the well can be cemented and shut off.

Image Ken Salazar, secretary of the interior, and David J. Hayes, right, the deputy secretary, attended a House hearing on Wednesday. Credit... Brendan Smialowski for The New York Times

Technicians guiding submarine robots completed diagnostic tests Wednesday morning and determined that the damaged blowout preventer and piping could withstand the pressure of the injection of thousands of pounds of heavy drilling fluids. The Coast Guard and federal agencies monitoring the effort agreed.

Minutes before the maneuver began, one technician working with BP said he was concerned that the effort could fail within minutes. Engineers had warned that the pressurized flow of liquids might not overcome the pressure of the oil and gas rushing from below the seabed.