On April 5, the IV Corps of Brigadier general Erasmus D. Keyes made initial contact with Confederate defensive works at Lee's Mill. This was an area McClellan expected to move through without resistance. Magruder's movement of troops back and forth convinced the Union that his works were strongly held. As the two armies fought an artillery duel, reconnaissance indicated to Keyes the strength of the Confederate fortifications, and he advised McClellan against assaulting them. McClellan ordered the construction of siege fortifications and brought his heavy siege guns to the front. In the meantime, Gen. Joseph E. Johnston brought reinforcements for Magruder.