Recon Zulu was the Marine squad from Earth that rescued the player at the end of Doom 3. The squad consisted of Marines and at least two Sentry Bots.
When the Fleet from Earth arrived at Mars in response to a distress call (sent by either the Protagonist or Dr. Betruger), Recon Zulu was one of the first teams sent to the planet's surface to investigate on the aftermath of the invasion. Upon entering Delta Labs, they found the entire facility in ruins, with all its personnel dead. Even more disturbing, they discover several skeletons, mutilated body parts, and words written in blood like "DIE", "BURN", "SUCCUMB" and "SUFFER". One of the Marines commented, "This place looks like Hell."
Recon Zulu eventually located a single survivor: a lone Marine. He was exhausted, but relatively unhurt. Recon Zulu quickly evacuated him off the base. They also located Counselor Elliot Swann who was dead, but were unable to locate Dr. Malcolm Betruger. Unknown to them, Betruger was in Hell and had been transformed into the Maledict.