In the predawn hours of 28 March sappers from the VC 409th Sapper Battalion approached the wire of FSB Mary Ann and took up positions to launch an attack. The exact number of sappers involved is uncertain, but most sources agree that there were at least 50. As was common practice for such units, the sappers wore khaki shorts and soot camouflage and were armed with either an AK-47 or RPG-7 and satchel charges and grenades to attack bunkers. Sappers relied on stealth, shock and surprise to give them an advantage, and rarely carried heavy weapons or equipment. The 409th was known to the intelligence section of the 196th LIB, but had previously operated against ARVN targets north of Quang Tin Province. It was assumed by the 196th LIB's intelligence personnel that both the 409th and the 402nd Sapper Battalions were east of FSB Mary Ann, preparing to attack ARVN targets in that region. None of them predicted an attack on the U.S. base.