(CNN) -- Tar balls found on a Florida Keys beach Monday, while not believed to be from a massive Gulf of Mexico spill, are nevertheless raising fears that oil will spread along the coastlines of Florida and beyond.

Researchers said it's unlikely -- although not impossible -- that the oil could have spread from the spill, off the coast of Louisiana, to the Keys so quickly. Additional tar balls were found on Keys beaches Tuesday, the Coast Guard said.

But they and federal officials seem to agree that a plume of oil is in the process of getting dragged into the Gulf of Mexico's Loop Current. The current flows through the Yucatan Channel between Cuba and Mexico, and then northward, where it loops southeast just south of the Florida Keys and travels to the west side of the western Bahamas, according to meteorologists.

A new tracking forecast prepared by four experts relying on five computer models shows that part of the oil may reach the Keys in five to six days and Miami five days after that, Sen. Bill Nelson, D-Florida, said in a statement Tuesday.

"While I always hope for the best, this is looking like really out-of-control bad," Nelson said.

The forecast cited by Nelson assumes the Loop Current's persistence and does not take into account dispersion and evaporation from the oil spill, CNN meteorologist Sean Morris said. It remains to be seen how much oil has entered the current. In addition, he said, "the Loop Current is much warmer than surrounding water, which would cause oil to evaporate at a greater rate than it is evaporating near the source of the spill."

More tar balls were found in the Florida Keys on Tuesday, on beaches in Big Pine Key and Loggerhead Key and on Smathers Beach in Key West, the Coast Guard said.

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The Coast Guard said in a statement that it responded to the Florida Park Service report of 20 tar balls on the beach at Fort Zachary Taylor State Park on Key West about 5:15 p.m. Monday.

Samples of the tar balls were sent to a laboratory for analysis to determine their origin. The Coast Guard and NOAA will conduct shoreline surveys of the area Tuesday, the statement said. An aerial search of the area with a pollution investigator is also planned. The Coast Guard did not speculate on the source of the tar balls.

An undersea oil well has been gushing an estimated 5,000 barrels of oil a day (210,000 gallons) into the Gulf since late April, when the drill rig Deepwater Horizon blew up and sank about 40 miles off Louisiana. Some estimates have put the amount of oil spewing from the well far higher.

Samples taken by scientists offshore have raised concerns that large plumes of oil are settling below the surface. But federal officials said the results have not been fully analyzed.

Some of the oil has washed ashore on the Louisiana coast. The Mississippi Emergency Management Agency said Tuesday that the slick was 50 miles off the Mississippi coast.

Meteorologist Jeff Masters said Tuesday that based on satellite imagery, "the oil is definitely in the Loop Current. ... The only question is how much oil made it in."

NOAA Administrator Jane Lubchenco said satellite imagery from Monday indicates that "the main bulk of the oil is dozens of miles away" from the Loop Current, but "a tendril of light oil has been transported down close to the Loop Current." The tendril's location shows that oil will probably be pulled into the Loop Current, if it has not already, she said.

Once in the current, the oil could reach waters near Florida in about eight to 10 days, she said.

Masters said that because parts of the Loop Current travel at speeds up to 4 mph, the earliest the oil could be near Florida is about four days.

Once the oil is in the Florida Straits -- waters between Florida, Cuba and the Bahamas -- onshore winds would be required to push it onto shore, along with oil getting into an eddy on the edge of the current, Lubchenco said.

During the time the oil travels to the area, she said, it would undergo the natural process of evaporation and dispersion and would change into tar balls.

Of the tar balls seen on Gulf Coast beaches thus far, some have been identified as stemming from the Deepwater Horizon spill, but others have not, Lubchenco said.

"I think it is safe to say that the tar balls washing ashore in the Florida Keys are an example of what might happen should oil become entrained in the Loop Current, and that that is the scenario that we will be anticipating and preparing for and will be tracking very, very carefully in terms of where the oil is relative to the Loop Current, and mobilizing resources," she said.

"The oil has been moved south by what we call a cyclonic eddy, an ocean eddy," said Nan Walker, an associate professor in Louisiana State University's Department of Oceanography and Coastal Sciences and director of its Earth Scan Laboratory. "It's as close as you can get without actually being in it."

The oil may not actually enter the Loop Current, as its speeds will keep it on the outside, but the current "is going to pull it all the way to Florida," Walker said.

However, the phenomenon does not explain the Key West tar balls, she said. It's possible that some oil previously moved eastward and became entrained in the Loop Current circulation. "Some of the shelf water has moved eastward and southward, but we can't actually see the oil [on images]," she said. "You can just see the motion of water."

Still, the timing "seems a little quick" for the oil to reach Florida, she said.

"It's a surprise," Chanmin Hu, associate professor of optical oceanography at the University of Miami, said of the tar balls.

Imagery currently shows the oil touching the current, he said, and it will be dragged in. But "to me, there's no way the oil can move this fast to reach the Florida Keys already."

He noted that "the Gulf is full of oil," with many wells, and "maybe there's some deepwater currents that bring oil from somewhere else." Still, he agreed with Walker that the possibility the tar balls stemmed from the spill cannot be ruled out.

Masters also said he doubted that the Keys tar balls are from the oil spill. "There's oil all the time in the Gulf anyway, just from natural seeps," he said.

The oil might not actually make it to Florida, depending on current patterns, he said. "The currents right now in the Gulf of Mexico are changing rapidly, day by day. It's kind of an unstable situation."

Hu said scientists "do not have a very reliable, sophisticated 3-D model to capture what's going on on the ocean bottom." Most efforts have been concentrated on the surface of the water, he said, and different depths have not been observed.

Hu said he and other researchers will travel on Wednesday for the spot where the oil meets the Loop Current and will collect samples and measure water properties. However, he said, "the mystery of the Florida Keys tar balls will remain."

According to the NOAA website, tar balls are actually remnants of oil spills. When crude oil floats on the surface of the water, it changes and is torn by winds and waves into smaller patches covering a wider area. The oil undergoes a process called "weathering."

The lighter components of the oil evaporate, and heavier components are left behind. Some crude oils mix with water to form "an emulsion that often looks like chocolate pudding" that is thicker and stickier than the initial oil, NOAA said. "Winds and waves continue to stretch and tear the oil patches into smaller pieces, or tar balls." The tar balls can travel hundreds of miles, according to the website.