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The U.S. Coast Guard removed 15 Cuban migrants from this bridge abutment. (Courtesy U.S. Coast Guard)

When 15 Cubans migrants set foot on a small piece of America after a perilous voyage across the Florida Straits, they were almost, but not quite, home free.

Their landfall on Jan 5 was at the base of a partly collapsed bridge, once part of the concrete backbone of the Florida Keys.

Applying what is called a "wet foot, dry foot" policy, U.S. officials declared they had not reached dry land in the United States. Cubans who reach dry land generally get to stay, while those stopped at sea do not.

"The historic Seven Mile Bridge runs side by side with a newer bridge," NBC2 News of Fort Meyers observed. "But it's missing several chunks – and the migrants had the misfortune of reaching pilings from an isolated section that no longer touches land."

On Monday, the U.S. Coast Guard Cutter Matagorda took the 15 men, women and children back to Cuba.

Their problem, the Miami Herald said, came down to "having landed on the wrong bridge piling."

A crossing on a homemade raft

They made the 170-kilometre crossing on a homemade raft, the Herald said, quoting a relative already living in Florida. The raft was sinking by the time they approached the Keys, so they landed the first place they could, the relative said.

Unmoved by pleas from Cuban-American groups, the government said they were in the same position as migrants intercepted at sea, of whom 52 were shipped back to Cuba on the same cutter voyage.

The bridge "is unused," the Coast Guard said in a statement on Monday, "and this segment is not connected to land. Through a legal review, the migrants were determined to be feet-wet and processed in accordance with standard procedure."