KUALA LUMPUR, Malaysia -- Nine months ago, authorities foiled an al-Qaida-linked plot to blow up the U.S. Embassy in Singapore using trucks loaded with explosives made from ammonium nitrate, a chemical fertilizer.

Dozens of Islamic militants were arrested, but the four tons of ammonium nitrate were never recovered and slipped from the public consciousness -- until the anniversary of the Sept. 11 attacks, when threats of strikes by al-Qaida closed embassies in Malaysia, Indonesia, Vietnam and Cambodia.

The feared weapon in the new strikes? Several tons of ammonium nitrate, U.S. intelligence indicated.

Authorities in Southeast Asia say that since the Sept. 11 attacks, they have broken the back of a regional extremist network with al-Qaida connections by detaining scores of suspects in Malaysia, the Philippines and Singapore.

But the trail of the missing ammonium nitrate, once thought to have been stored in Malaysia, has gone cold.

Malaysian police believe they traced the stockpile to Batam island, an outcropping in sight of Singapore that is Indonesian territory and therefore outside the jurisdiction of Malaysian officials.

"As far as we know, the ammonium nitrate was sent to Batam," a Malaysian official said, speaking on condition of anonymity. "We don't know what happened after that."

Widely available as fertilizer, ammonium nitrate becomes an explosive more powerful than dynamite when mixed with fuel oil.

Timothy McVeigh used two tons of the fertilizer -- mixed in barrels and loaded into a rental truck -- to bomb the federal building in Oklahoma City in 1995, killing 168 people.