Washington (CNN)-- BP finished pouring cement down its crippled well in the Gulf of Mexico on Thursday in an operation known as a "static kill," completing the job earlier than expected.
The government official overseeing the effort is sounding increasingly optimistic that the end is in sight in the drive to seal the well once and for all.
BP began pouring cement into its undersea well in the Gulf of Mexico at 9:15 a.m. ET Thursday, and it finished pouring it at 3:15 p.m. That followed 2,300 barrels of heavy drilling mud, poured down from a ship on the surface Tuesday.
Before word came that the cementing had been completed, retired Adm. Thad Allen said the development would amount to a "significant milestone" in the long-running fight against the BP oil spill.
He said the cementing phase of the "static kill" operation "is not the end" of the process, "but it will virtually assure us there's no chance of oil leaking into the environment."
"We will have created a significant milestone and made a major step forward probably by tomorrow [Friday] when the cementing is done," Allen told reporters. Later, BP announced the work had been finished.
"I think we can all breathe a little easier regarding the potential [that] we'll have oil in the Gulf ever again," Allen said. "But, we need to ensure the people of the Gulf and the people of the United States that this thing is properly finished and that will be through the bottom."
While the cementing could potentially plug the well for good, Allen is instructing BP to proceed with a final "bottom kill." That's when more mud and cement will be poured into the well through one of two relief wells. The bottom kill amounts to an insurance policy.
Two relief wells are being drilled so one can act as a backup if the first one misses the mark. The main well is about 100 feet from BP's oil well, and the drilling job is expected to be completed in mid-August.
The advantage of the bottom kill is that the relief well will penetrate the drill pipe's two layers -- both an outer casing and inner piping -- and then the mud and cement will fill both portions. That would guarantee against any leakage into the Gulf.
In the static kill, there is no assurance cement would fill the void between the two layers, known in oil-well terminology as an "annulus."
Despite the optimistic notes being sounded about killing the well, the long-term cleanup is another matter.
The federal government's on-scene coordinator, Rear. Adm. Paul Zukunft, said Thursday it's possible that tar balls may be washing ashore for years.
"In terms of tar balls washing ashore, we are definitely talking about months, potentially years," he told reporters. "We'll be responding to tar balls as long as necessary and then holding the responsible parties accountable. Probably, well, I'll just say, for the next year I expect to see episodic tar balls."
A government report released this week concluded that of the nearly 5 million barrels of crude oil that spilled into the Gulf when the well was out of control, roughly a quarter of it remains in the water either on or just below the surface as a light sheen and weathered tar balls.
The rest has been collected by skimming ships, burned on the surface, broken up by dispersants or evaporated.
The tar balls are either washing ashore, being collected from the coastlines, or buried in sand and sediment and are in the process of being degraded, the report said. Zukunft said federal authorities are "in this for the long term." "The permanent kill of this well only brings us to another phase. The restoration of the Gulf is really the long-term objective here."
Zukunft spoke on the 108th day since the environmental disaster started with the explosion on the Deepwater Horizon that sank the rig and killed 11 workers. Oil flowed relentlessly for nearly three months, as BP tried a variety of methods to cap the runaway well, until it finally placed a tightly fitting cap on it in mid-July. That opened the door for the efforts now under way to permanently seal it.
Long-range efforts to restore coastal areas were also in focus Thursday, when the federal point man on that effort, Secretary of the Navy Ray Mabus, toured Gulf areas with Louisiana Gov. Bobby Jindal.
In addition to meeting with Jindal, Mabus is holding a series of town hall meetings with coastal residents.
"We want to make sure that whatever comes forward, comes from the coast, from the people who live here and work here and make their lives here," Mabus said after meeting Jindal.
Later, as part of that effort, Mabus held another in a series of town hall meetings with coastal residents.
Speaking in Buras, Louisiana, Mabus told a packed auditorium, "This is not like Hurricane Katrina. This is like slow motion. This is like a disaster that keeps happening over and over again. And we have to make sure the rest of the country doesn't forget about us when the oil spill is over."
Mabus was speaking as a native. He is a fourth-generation Mississippian who served as the state's governor from 1988 to 1992.
He heard plenty of frustration at the town hall meeting, including from Marla Cooper, a council member from a southern district in Plaquemines Parish who represents many fishermen.
"We cannot even begin to start our recovery, because we don't know the extent of the damage to our industry, so it's going to be a hard task," she said. "A lot of these people aren't coming up to talk because we don't know what to do at this point. We're trying to survive day by day."
Some partipants were of Vietnamese descent and spoke through an interpreter.
Robert Huynh said he has lived in the area since 1986 and has been teaching his six children about commercial fishing.
"We have the suspicion that BP may want to get out of this region," he said. "The oil is disappearing from the surface, but it is going to sink and dirty the water. In our profession, we don't fish for shrimp on the surface, but we have to comb under the water for the shrimp."
"We have relied on the fishing for our livelihood," he continued, "but lately BP has a tendency to avoid the responsibility, and the compensation is not adequate."
BP said Thursday it has paid $303 million in claims payments so far to more than 40,000 individuals and businesses affected by the spill. The company said it is trying to smooth the transition for later this month, when claims will be handled by the new Gulf Coast Claims Facility run by Kenneth Feinberg. He will administer the $20 billion fund that BP executives agreed to provide in a meeting with President Barack Obama in June.
Another man in the town hall meeting scoffed at the idea that oil is disappearing, as he waved a soda bottle containing a dark liquid. He said it was oil he had collected from a Louisiana bay.
"How can seafood be safe?" he asked, to applause. "I accidentally found this oil in one of our bays. We are going to be in deep trouble, because this is in our bays, right now.
"For anybody to tell us that the oil is 75 percent gone, when it's right here. Look at this bottle. This is in our bays, and we're going to open our season to fishing? We're going to have this in our shrimp when we start dragging? We're going to go to recovery mode? We ain't through cleaning. It's here."