In a stinging assessment, congressional auditors say the Homeland Security Department has fallen far short of meeting its performance expectations in the first four years of its existence.

The department's primary mission is to prevent another terrorist attack and to minimize the damage should an attack take place. However, maritime security is the only area in which significant improvements have been made, the draft report by the Government Accounting Office said.

The report, which was being issued Thursday, measured progress in 14 mission and management areas. Auditors identified 171 performance expectations and found that the department had achieved fewer than half of them.

The department has had success developing plans but has not always been able to carry out the programs, such as its efforts to identify places where people illegally enter the United States, the report said.

The department disagreed with the GAO findings, particularly with the way auditors defined and measured progress.

In a July 20 letter to Comptroller General David Walker, the department's undersecretary for management, Paul A. Schneider, accused the auditors of shifting "already nontransparent" performance measures during their evaluation.

"The GAO report is based on a flawed methodology that results in an inaccurate representation of the department's progress and fails to accurately reflect the department's progress in many specific program areas," Schneider wrote.

During congressional testimony Wednesday, Homeland Security Secretary Michael Chertoff defended the department's progress and assured lawmakers the country was safer than it was before the Sept. 11, 2001, terrorist attacks.

"But if you ask me is the job of keeping us safe done, the answer to that is no," Chertoff said. "It is not done. And it may not be done within our lifetimes."

The department was formed out of 22 federal agencies in 2003, and since then GAO has made nearly 700 recommendations for mission and management changes. Experts, including GAO auditors, have said it would take a department this large between five years and seven years to come together.

Auditors also note that the Defense Department, created 50 years ago, still faces serious management challenges.