Models engaged in close combat may not fire weapons in the shooting phase. Their attentions are completely engaged by the swirling mêlée. Likewise, while especially twisted and soulless commanders may wish their warriors to fire indiscriminately into close combats in the hopes of hitting the enemy, this is not permitted. The events in a close combat move too quickly and the warriors themselves will be understandably hesitant about firing on their comrades (they may end up in the same straits soon enough after all).
Models belonging to Locked units that are not engaged in combat themselves, or within 2" of engaged friends, may shoot normally in their own shooting phase and may be shot at in the enemy shooting phase. Blast markers can be re-positioned subject to the normal rules — however it is probably not a good idea to fire a plasma cannon into the back of a close combat if you value the combatants much.