WASHINGTON (AP) -- The National Academy of Sciences is casting more doubt on whether the Obama administration's European-based missile defense shield can protect the United States and recommends scrapping key parts of the system.

The conclusions from the academy, which advises the government on science and technology, were contained in a letter to lawmakers obtained by The Associated Press. The academy's assessment could complicate the Obama administration's efforts to persuade Congress to fund the still-developing program.

Though the academy says the plan would protect Europe effectively, some lawmakers already are asking why the U.S. should spend billions of dollars on a system that provides limited homeland defense.

The academy's letter bolsters two earlier reports by Defense Department advisers and congressional investigators that said the system faced significant delays, cost overruns and technology problems.