The Z-Sec (Zombie-Security) is a closer recreation of the classic former humans, as they carry firearms; Doom 3's civilian Zombies do not. Unlike the classic variations, all Z-Secs wear black uniforms and armor, excluding the pistol guard, possibly as reference to the classic Shotgun Guy. In contrast to the classic former humans, which were previously government marines, Z-Secs were originally UAC security guards (private security contractors) that were possessed and mutated by evil demonic spirits.
The base's Marine contingent appears to have mostly avoided this zombification fate, but were soon overrun by demonic forces; the player witnessed one marine being strangled to death by a pistol-wielding Z-Sec in a communication terminal shortly before another Z-Sec appears to attack him. In trailers and early in the game, an in-game monitor shows one being possessed by a ghostly skull, which causes his skin to turn white and his eyeballs to explode.
There are four types of Z-Secs, each with its own distinct characteristics and firearms. These are the Pistol Guard, the Shotgun Guy, the Machine Gunner and the Riot Shield Trooper.
A sure sign that the player is about to be attacked by Z-Secs is the telltale radio chatter that accompanies them. The radio chatter itself is mostly low growls and groans, as well as a few repeated phrases and it doesn't seem as though the Z-Secs rely on radio communications to either organize themselves or work together.
Unlike regular Zombies, Z-Secs do not suffer from slow movement, and possess the same speed and agility as a normal human soldier. However, they run noticeably slower than the player does. They are also notable for their artificial intelligence, which makes them fight like soldiers, by securing and maintaining their position or closing in to the player to engage in an attack. When shot, they can sometimes sidestep a single step in an attempt to dodge further attacks. However, they do not shoot and move at the same time.
- Pistol Guard: Similar to the Zombieman from the Classic Dooms. He is a bald man with no helmet and glowing white eyes, armed with a pistol. They have the unique ability to dodge by rolling sideways, and always retaliate with a few pistol shots after a successful dodge. They occasionally fire multiple shots in rapid succession and are generally decent shots, but of all three Z-sec types, these are the ones that are the most likely to miss. They have about half as much health as the other Z-sec types. Drops a Pistol when killed.
- Shotgun Guy: Similar to the Classic Doom Shotgun Guy, but he wears a helmet with a broken visor, and completely hollow eye sockets. Unlike the player's shotgun, the Z-sec shotgunner's weapon has narrower spread, but it is considerably less powerful than the player's version. Can sometimes rapid-fire at a faster rate than the player. Drops a Shotgun when killed.
- Machine Gunner: Wears a closed helmet with an intact but blood-spattered visor (obscuring his eyes) and a breather, ultimately covering his entire face. Unlike shotgun Z-Secs, instead of rushing the player's position, these zombies will often dig in behind cover and aggressively spray the player's position with bullets. This Z-Sec type is the only one that can fire and run straight at the player at the same time. If the player hides behind cover, these Z-Secs will suppress them with continuous machine gun fire. Drops a Machine Gun when killed.
- Riot Shield Trooper: The rarest Z-Sec type, appearing only in one level (Central Communications Tower). Has the same appearance as the Machine Gunner. In addition to the Pistol, this type carries a large riot shield, which is impervious to any firearm projectiles. This Z-Sec will also occasionally kneel down while holding it's shield to make itself less available as an easy target. Drops a Pistol when killed.
Although Doom 3 Z-Secs serve a cannon fodder role when compared to the game's tougher demonic enemies, they are comparatively tougher than the classic former humans. They are equipped with better AI, making them more focused and intelligent. They can dodge, take cover, and advance towards the player, instead of wandering semi-randomly. They also have better aim, are a lot more aggressive, and fire more frequently.
Like most of Doom 3's monsters, headshots do twice as much damage as regular body shots. Compared to their Classic Doom counterparts,
The presence of Z-Secs is usually announced before you actually see them due to the fact that their radios are still working; you can hear garbled radio chatter from their suits seconds before you see them.
Using cover is extremely important when fighting these Zombies, as their hitscan weapons cannot be dodged by circle strafing. The Machine Gun is generally a good basic weapon and a good tactic is usually to mow them down with concentrated pistol or machinegun fire, switching to the shotgun to finish them off if they rush the player at close range.
It is usually a bad idea to use the shotgun right away as it has short range and the Z-Secs are tough enough to potentially survive anything less than a direct point-blank blast, leaving players vulnerable after the initial shot. However, a headshot with the shotgun will kill a Z-Sec instantly. With a little practice, it is relatively easy to ambush Z-Secs by lurking just behind a corner and killing them with a point-blank shotgun blast when they come around the corner to chase you. If they hide behind cover, grenades are also a viable option.
- Pistol Guard: The Pistol works well against this type, as it take four bullets to the body to kill him. It is often a good idea to keep a good distance as he is more likely to miss his mark when the player is far away. Other weapons like the Shotgun and Machine Gun are also effective are taking them down from close range or long range, respectively.
- Shotgun Guy: This type should be brought down as quickly as possible, as he is capable of dealing a great deal of damage if left unchecked. Using the machine gun to soften him up at range and then finishing him off when he gets in close is an effective strategy, but caution should be taken to only come out of hiding after he has fired a shot. More powerful weapons should be considered. Running straight at him is definitely not recommended, as you risk yourself to his highly damaging attacks. Alternatively, hide behind a doorway when available, shotgun at the ready. Wait for him to charge through the door, then blast him in the face. This works most of the time, but be prepared to fire again if they survived the first shot.
- Machine Gunner: Although he fires more than the other types, his bullets are less powerful. Patience is a good tactic, as he tends to pin down the player with prolonged sprays. It is effective to wait for him to come out of hiding, shoot and quickly duck down behind cover before he opens up. It is a bad idea to fire prolonged bursts at him, as he is tough enough to recover swiftly from hits, often enabling him to retaliate swiftly. Machine Gunners do not fire and move at the same time, and so they are vulnerable to return fire when advancing forward towards the player's position. Alternatively, you can use the same doorway tactic as with the Shotgun Guy. This tactic works better against Machine Gunners because even if he survives the first shot, he will take the time to kneel before shooting you, allowing you to get in a finishing shot.
- Riot Shield Trooper: The best way to dispatch this Z-Sec is to run into him with the Chainsaw (it will damage him even when the shield is hit). Another possibility is to hit him with a grenade or with a rocket.
- When you pick up the PDA of Seamus Blake in Communications, one of his E-mails is a receipt for a CD of music called "Champagne Tomiko" by "The ZSecs."
- The Riot Shield Trooper's Pistol uses the same gunshot audio as the player's Pistol. Curiously, this does not apply to the Pistol-wielding Z-Sec's weapon, despite being the same thing.