Western governments and nongovernment organizations have repeatedly asked the National Transtional Council, the government of Libya's former rebels, to secure the vast stockpiles of arms that the de facto authority has inherited, apparently to little avail.

TRIPOLI, Libya — The sign on the wall reads “Schoolbook Printing and Storage Warehouse,” but the double gates have been ripped off, suggesting something more interesting might be inside.

It turns out the only books in any of the three large buildings in the walled compound are manuals: how to fire rocket launchers and wire-guided missiles, among others. The buildings are disguised warehouses full of munitions — mortar shells, artillery rounds, anti-tank missiles and more — thousands of pieces of military ordnance that are unguarded more than two weeks after the fall of the capital, Tripoli.

Perhaps most interesting is what is no longer there: shoulder-fired heat-seeking missiles that could be used by terrorists to shoot down civilian airliners. U.S. authorities have long been concerned Libyan missiles could easily find their way onto the black market.

These missiles, mostly SA-7b Grails, as NATO refers to them, have been seen in Libya before and are well known to have been sold to the government of Moammar Gadhafi by former Eastern bloc countries.

The evidence at the schoolbook warehouse raises questions about how many may have been stolen by rebels, criminals or smugglers.

Matthew Schroeder, who researches heat-seeking anti-aircraft missiles and their proliferation for the Federation of American Scientists, said the discovery of another looted arms depot in Libya was cause for concern,especially depots that contained what security specialists call Man-Portable Air-Defense Systems, or Manpads.

Western governments and nongovernment organizations have repeatedly asked and prodded the rebel government, the National Transitional Council, to secure the vast stockpiles of arms that the de facto authority has inherited, apparently to little avail.

“Claims that depots holding Manpads and other dangerous weapons are still not being properly secured are very worrisome and should be thoroughly investigated,” Schroeder said.

In Washington, President Obama’s top counterterrorism official, John Brennan, said the spread of shoulder-fired missiles and other weapons from Libya’s arsenal posed “a lot of concern,” and the United States had pressed the rebel government to secure stockpiles of weapons.

“Obviously, there are a lot of parts of that country right now that are ungoverned,” Brennan said at a security conference.

Wednesday, three reporters and a researcher for Human Rights Watch who visited the scene found 10 crates that had held two missiles each lying opened and empty. The crates were clearly labeled as coming from Russia.

“Other countries know these weapons are on the loose, and they will be trying to get their hands on them,” said the researcher for Human Rights Watch, Peter Bouckaert.

He was particularly concerned with one crate, labeled “9M342,” which is the Russian designation for the SA-24 heat-seeking missile.

“These were some of the most advanced weaponry the Russians made,” Bouckaert said.

A spokesman for the Libyan rebel military, Abdulrahman Busin, said the rebel authorities were aware of the schoolbook warehouse.

Business said the rebel “military police” had probably removed the missiles.

He was unable to explain why the facility remained unguarded. Efforts were unsuccessful to contact the head of the military police to confirm if his forces had the missiles.