(AP) As the government and the people of New York City wrestle with the question of how to rebuild at Ground Zero, federal officials are about to launch a two-year, $16 million investigation of the World Trade Center collapse on Sept. 11.

The probe by the National Institute of Standards and Technology could be hampered by the Senate's failure to pass legislation that would give federal building investigators subpoena power and other tools. The National Construction Safety Act, sponsored by Rep. Sherwood Boehlert, R-N.Y., passed the House in July but has stalled in the Senate Commerce Committee.

In the aftermath of the trade center collapse, investigators struggled to gain access to the site and to key documents, like blueprints.

The NIST probe set to begin this week is designed to be broader and more detailed than a study conducted by the American Society of Civil Engineers and funded by the Federal Emergency Management Agency. That study, concluded in May, determined that the 110-story twin towers could have survived the impact of the hijacked 767s but fell victim to the ensuing fire that lit office furniture and paper ablaze. That intense fire caused the buildings' steel columns to soften and buckle.

NIST Director Arden Bement has said the relationship between fire and structural collapses will be a major focus of the probe and the investigation "could lead to major changes in both U.S. building and fire codes and in engineering practice."

Fluffy fireproofing sprayed onto the trade center's steel beams was jarred loose by when the jetliners slammed into the trade center, the FEMA study found. NIST investigators will look at ways to keep the fireproofing intact.

Investigators are also interested in 7 World Trade Center, which is believed to have sustained little structural damage. It collapsed due to fire alone, the first fireproofed steel structure to do so.

NIST will also examine ways to harden exit stairways to make them less vulnerable to severe impact and plans to space those stairways out so one blow might not render them all impassable. Such designs might have allowed occupants to have escaped from the floors above where the planes hit.

Glenn Corbett, assistant professor of fire science at John Jay College of Criminal Justice in New York, called NIST's undertaking "a landmark event."

"This is going to be the most extensive building disaster investigation ever performed," Corbett said. "The size of the disaster dictates that it has to be."

But he said he was worried that without subpoena power the investigation could be limited.

Corbett and others were critical of the ASCE probe, saying it was too narrow in scope. He and others also complained that officials lost a crucial opportunity to examine most of the trade center's steel beams for clues, allowing them instead to be recycled.

Some of the remaining steel beams are already at NIST's Gaithersburg, Md., headquarters to be examined as part of the new probe.