ON THE GULF OF MEXICO – Icelike crystals encrusting a 100-ton steel-and-concrete box meant to contain oil gushing from a broken well deep in the Gulf of Mexico forced crews Saturday to back off the long-shot plan, while more than 100 miles away, blobs of tar washed up at an Alabama beach full of swimmers.

The failure in the first attempt to use the specially constructed containment box over the leak 50 miles off the Louisiana coast, coupled with the arrival of the sticky substance at Dauphin Island, Ala., crushed hopes of a short-term solution to what could yet grow into the worst oil spill in the nation’s history.

More than 3 million gallons of crude have spewed into the Gulf since a rig exploded April 20, killing 11, and officials said it would be at least Monday before a different solution is found.

Authorities in protective gear descended on the public beach on Dauphin Island, three miles off the Alabama mainland at the mouth of Mobile Bay and much farther east than oil had been reported.

Kimberly Creel, 41, was hanging out with hundreds of other beachgoers when crews arrived to investigate. She said she found quarter- to pea-size balls sporadically along the beach.

“It almost looks like bark, but when you pick it up, it definitely has a liquid consistency and it’s definitely oil,” she said.

A half-dozen tar balls had been collected by Saturday afternoon at Dauphin Island, Coast Guard Chief Warrant Officer Adam Wine said.

The containment box, a method never before attempted at such depths, had been considered the best hope of stanching the flow in the near term.

“I wouldn’t say it’s failed yet,” said Doug Suttles, BP chief operating officer. “What I would say is what we attempted to do last night didn’t work.”

The icy buildup on the containment box made it too buoyant and clogged it up, Suttles said. Workers who had carefully lowered the massive box over the leak nearly a mile below the surface had to lift it and move it some 600 feet to the side.

Company and Coast Guard officials had cautioned that icelike hydrates, a slushy mixture of gas and water, would be one of the biggest challenges to the containment box plan, and their warnings proved accurate. The crystals clogged the opening in the top of the peaked box like sand in a funnel, only upside-down.

Options under consideration included raising the box high enough that warmer water would prevent the slush from forming, or using heated water or methanol to prevent the crystals from forming.

Even as officials pondered their next move, Coast Guard Rear Adm. Mary Landry said she must continue to manage expectations of what the containment box can do. “This dome is no silver bullet to stop the leak,” she said.

A petroleum chemist and geologist said the icy hydrates are a common problem at depths greater than 1,200 feet.

“At this water depth and the nature of the containment vessel, it’s hard to know what their options really are,” said Art Johnson of Kenner, La., president and chief of exploration of Hydrate Energy International.

The original blowout was triggered by a bubble of methane gas that escaped from the well and shot up the drill column, expanding quickly as it burst through several seals and barriers before exploding, according to interviews with rig workers conducted during BP PLC’s internal investigation.

While the precise cause is still under investigation, the sequence of events described in the interviews provides the most detailed account of the blast.

Portions of the interviews — two written and one taped — were described in detail to the Associated Press by Robert Bea, a University of California Berkeley engineering professor who serves on a National Academy of Engineering panel on oil pipeline safety and worked for BP PLC as a risk assessment consultant during the 1990s. He received them from industry friends who were seeking his expert opinion.

BP executives were on board the Deepwater Horizon rig celebrating the project’s safety record, according to the transcripts. Far below, the rig was being converted from an exploration well to a production well.

Based on the interviews, Bea said he believes that the workers set and then tested a cement seal at the bottom of the well. Then they reduced the pressure in the drill column and attempted to set a second seal below the sea floor. A chemical reaction caused by the setting cement created heat and a gas bubble that destroyed the seal.

Deep beneath the seafloor, methane is in a slushy, crystalline form. Deep-sea oil drillers often encounter pockets of methane crystals as they dig into the earth.

As the bubble rose up the drill column from the high-pressure environs of the deep to the less pressurized shallows, it intensified and grew, breaking through safety barriers, Bea said.

“A small bubble becomes a really big bubble,” Bea said. “So the expanding bubble becomes like a cannon shooting the gas into your face.”

Up on the rig, the first thing workers noticed was the sea water in the drill column suddenly shooting back at them, rocketing 240 feet in the air, he said. Then, gas surfaced. Then oil.

“What we had learned when I worked as a drill rig laborer was ‘swoosh, boom, run,’” Bea said. “The swoosh is the gas, boom is the explosion and run is what you better be doing.”

On Saturday, the boat with the plumbing equipment for the containment box was about 1.5 miles from the vessel that lowered the box. Company officials had hoped the box would capture up to 85 percent of the oil and feed it into a tanker on the surface.

A sheen of oil began arriving on land last week, and crews have been laying booms, spraying chemical dispersants and setting fire to the slick to try to keep it from coming ashore. But now the thicker, stickier goo — arrayed in vivid, brick-colored ribbons — is drawing ever closer.

In New Orleans, about 200 people showed up at a Sierra Club rally, speaking out against the dangers of offshore drilling. Protesters, some holding signs with slogans such as “Clean, Baby, Clean” and “Save Our Wetlands Now,” signed a massive banner that read: “This Is Your Crude Awakening.”