Since then, the government has faced questions about why it took so long to declare the spill an emergency, why it didn't use Pentagon planes sooner to spray dispersants, and why it didn't have a ready supply of specialized booms to contain and burn the growing oil slick.
And both industry and federal officials have been forced to scramble to develop on-the-fly, untested technological solutions — such as concrete and steel containment domes and well plugs made of shredded tires and golf balls — to try to contain the epic leak on the sea floor, more than a mile under water.
Oil industry officials told members of Congress Tuesday that new technology is being used to combat the ongoing spill. In response to questions, they identified the use of dispersants to attack the spill under water as a new approach that had been honed over the past several years. But they also acknowledged that a spill at this depth has presented them with problems they weren't prepared for.
"I think, after this is under control and thought about in hindsight, there will be some ideas about how to make the subsea intervention and response better," said BP America's chairman and president, Lamar McKay. "I think we're learning right now as we go."
U.S. officials said in interviews that the elaborate dry runs taught them important lessons that are making the ongoing response stronger and more effective. But they also acknowledged that they have yet to resolve some of the persistent problems related to communication, coordination and technology – problems that surfaced during the drill conducted March 24-25 in New England, simulating a response to an oil tanker leaking 18 million gallons of crude after a collision off the coast of Maine.
"Every exercise you do, you come out with the question of whether your communication skills are up to the challenge," Coast Guard Lt. Kelly Dietrich said in an interview.
But Dietrich said the Coast Guard and its federal allies have made steady improvement through the training exercises and don't deserve some of the criticisms that have been raised by lawmakers and residents in the Gulf.
"We always go out with full force," she said. "It always seems to the public that it seems slow because most people aren't involved in the preparatory work."
For more on the government's preparations for a major oil spill off the U.S. coast, visit the Center for Public Integrity.
John Solomon and Aaron Mehta are reporters with the Center for Public Integrity, a nonprofit investigative journalism organization.
ABC News' Asa Eslocker contributed to this report. Click Here for the Blotter Homepage.