Tar balls found on a Texas beach were confirmed as the first evidence that gushing crude oil from the Deepwater Horizon well had reached all the Gulf states.
The US Coast Guard said it was possible that the oil had attached itself to a ship and was not carried naturally by currents to the barrier islands of the eastern Texas coast, but there was no way to know for sure.
The amount discovered is tiny in comparison to what has coated beaches so far in the hardest-hit parts of the Gulf coast in Louisiana, Mississippi, Alabama and the Florida Panhandle. It still provoked the quick dispatch of cleaning crews and a vow that BP will pay for the trouble.
"Any Texas shores impacted by the Deepwater spill will be cleaned up quickly and BP will be picking up the tab," Texas land commissioner Jerry Patterson said.
The spill began after an April 20 explosion on the Deepwater Horizon killed 11 workers and started the worst oil spill in Gulf history.
The oil's arrival in Texas was predicted on Friday by an analysis from the National Oceanic and Atmospheric Administration, which gave a 40% chance of crude reaching the area.
"It was just a matter of time that some of the oil would find its way to Texas," said Hans Graber, a marine physicist at the University of Miami and co-director of the Centre for Southeastern Tropical Advanced Remote Sensing.
About five gallons of tar balls were found on Saturday on the Bolivar Peninsula, north east of Galveston, said Captain Marcus Woodring, the coastguard commander for the Houston/Galveston sector. About two gallons were found on Sunday on the peninsula and Galveston Island, though tests have not yet confirmed their origin.
Capt Woodring said the consistency of the tar balls indicated it was possible they could have been spread to Texas water by ships that had worked out in the spill, but there was no way to confirm the way they got there.
Galveston mayor Joe Jaworski said he believed the tar balls were a fluke rather than a sign of what's to come.
"This is good news," he said. "The water looks good. We're cautiously optimistic this is an anomaly."
The distance between the western reach of the tar balls in Texas and the most eastern reports of oil in Florida is about 550 miles. Oil was first spotted on land near the mouth of the Mississippi River on April 29.
The spill is reaching deeper into Louisiana. Strings of oil were seen yesterday in the Rigolets, one of two waterways that connect the Gulf with Lake Pontchartrain, the large lake north of New Orleans.
"So far it's scattered stuff showing up, mostly tar balls," said Louisiana Office of Fisheries assistant secretary Randy Pausina. "It will pull out with the tide, and then show back up."
He said he expected the oil to clear the passes and move directly into the lake, taking a back-door route to New Orleans.
The news of the spill's reach comes at a time that most of the offshore skimming operations in the Gulf have been halted by choppy seas and high winds. A tropical system that had been lingering off Louisiana flared up last night, bringing heavy rain and winds.
Forecasters at the National Hurricane Centre said there was a 60% of the storm becoming a tropical cyclone.
Skimming operations across the Gulf have scooped up about 23.5 million gallons of oil-fouled water so far, but officials say it is impossible to know how much crude could have been skimmed in good weather because of the fluctuating number of vessels and other variables.
BP has now seen its costs from the spill reach £2.1 billion, a figure that does not include a £13.2 billion fund for damages the company created last month.
The storms have not affected drilling work on a relief well that BP says is the best chance for finally plugging the leak. The company expects drilling to be finished by mid-August.