WASHINGTON -- Despite spending nearly $3 billion last year to stop terrorists from bringing nuclear materials into the United States, the federal government lacks a comprehensive strategy to prevent nuclear terrorism, an investigator told a Senate hearing Wednesday.

The Domestic Nuclear Detection Office, which is charged with preventing a nuclear terrorist attack, “has not developed an overarching strategic plan,” said David Maurer of the Government Accountability Office, an arm of Congress.

Homeland Security’s detection office was created in 2005 to design a worldwide system monitoring nuclear and radioactive materials, including spent fuel from power reactors, industrial waste and certain substances used for cancer treatment.

Their goal is to prevent the detonation of a nuclear weapon or a dirty bomb in the U.S. The office coordinates 74 federal programs, ranging from securing radioactive sources in other countries to scanning sea freight at American ports. The programs spent a combined $2.8 billion last year.

The task of keeping nuclear materials out of terrorists’ hands is “our first priority,” said Joseph Lieberman, chairman of the Homeland Security and Governmental Affairs Committee. The Connecticut independent has held a series of hearings in recent months examining the threat of nuclear terrorism.

Robert F. Nesbit, a member of the Defense Science Board, a Pentagon advisory group, underscored the importance of preventing terrorists from acquiring such materials in the first place.

“If a terrorist got his hands on a nuclear device,” he told the committee, “that shows a level of skill way beyond the normal terrorist that we deal with. And if they have that level of skill, expertise, financing to be able to get their hands on the device, they could be really clever about how they got it into the U.S.”

In the past three years, the detection office has identified and started to fill gaps in the programs it oversees, Assistant Director Mark Mullen said. Mullen cited pilot projects under way in the Puget Sound, San Diego and New York to expand radiation monitoring to small boats, which experts have identified as a potential smuggling method.

The GAO’s Maurer agreed that such programs “appear to be a step in the right direction,” but said the office “lacks a strategic roadmap that would clearly establish goals, responsibilities, resource needs, and mechanisms for assessing projects along the way.”

The detection office did not dispute the GAO’s findings.

But the cost of implementing such a plan would go far beyond last year’s $2.8 billion expenditure, and would “likely cost billions of dollars, take several years and rely on the expertise and resources of agencies and programs across the government,” Maurer said.