The Homeland Security Department has yet to produce a plan to protect the U.S. transportation system 3 1/2 years after the Sept. 11 terrorist attacks, and at least two senators are getting impatient.

Sens. Susan Collins of Maine and Joe Lieberman of Connecticut released a letter Friday pressing Homeland Security officials to finish a transportation security plan within the next three months. Collins, a Republican, chairs the Homeland Security and Governmental Affairs Committee and Lieberman is the panel’s ranking Democrat.

“We are concerned that the delay in producing a cohesive and comprehensive transportation security strategy is adversely affecting the federal government’s ability to protect the citizens of the country,” said the letter, dated Wednesday and addressed to Homeland Security Deputy Secretary Michael Jackson.

Though intelligence officials say al-Qaida continues to be interested in attacks on aviation, terrorists have also attacked ships, trains, transit systems and buses around the world.

Under questioning from Congress in 2003, then-chief of the Transportation Security Administration James Loy said a plan would be finished by the end of the year. Congress later set a deadline of April 1 for the plan. It was not met.

The new Homeland Security secretary, Michael Chertoff, is reviewing the department’s plans and policies, said spokeswoman Kathleen Montgomery.

“In the process of that review, the secretary and Deputy Secretary Jackson determined significant additional work is needed to ensure the strategy is consistent with other strategic planning efforts underway in the department,” Montgomery said.

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