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. Several of the Z-Secs have their eye sockets hidden due to their helmet visor, although it is assumed that they have lost their eyes. The visor itself is completely red, probably due to the blood. Despite this, they are still able to "see" the player, possibly due to the hellish spirits controlling them.
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.
Unlike regular zombies, Z-Secs do not suffer from slowed movement, and possess the same speed and agility as a normal human soldier. They are also notable for their artificial intelligence (AI), which makes them fight like soldiers. They duck behind objects, lean around or over cover to take shots at the player, and occasionally rush the player's position. When shot, they can sometimes sidestep a single step in an attempt to dodge further attacks (this can effectively allow them to dodge a follow-up shotgun blast at close range). However, they do not shoot and move at the same time, leaving them vulnerable to the player's fire when advancing forward from one position to another. They also run noticeably slower than the player does, making them relatively easy to hit when not hiding behind cover. A sure sign that the player is about to be attacked by Z-Secs is the telltale radio chatter that accompanies them, perhaps a homage to Half-Life Grunts (government soldiers). The radio chatter itself is mostly low growls and groans, as well as a few repeated phrases (including one that sounds like "Beethoven"), and it doesn't seem as though the Z-Secs rely on radio communications to either organize themselves or work together.
Like most of Doom 3's monsters, headshots do twice as much damage as regular body shots. Compared to their Classic Doom counterparts, Z-Secs are more focused and intelligent, as 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.
- Pistol Guard: Similar to the Zombieman from the Classic Dooms. Bald guy 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, and take around four pistol bullets to the body to kill. 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. He is about twice as tough as the Pistol Z-Sec, but still reasonably easy to kill. 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. Arguably the toughest of the three types. Drops a Machine Gun when killed.
- Riot Shield Trooper: The rarest Z-Sec type, with only a few being encountered throughout the game (and not encountered at all in the Xbox version of the game). Has the same appearance as the Machine Gunner. In addition to a pistol, this type carries a large riot shield, protecting it from attacks. The shield effectively protects it from anything that hits the shield, and being fully armored himself, he is still fairly tough (similar to the Machine Gunner) when hit where his shield does not protect him. Players are advised to use grenades against them, as the shield the Z-Sec carry does not protect itself from the explosive damage from Grenades. AI is the standard Z-Sec combat tactics. The Pistol sound is just like the player's.
Although Doom 3 Z-Secs serve a cannon fodder role when compared to the game's tougher demonic enemies, they are both comparatively tougher than the classic zombie troops and equipped with better AI, and should not be underestimated.
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 against these pests 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, but the machine gun gets the job done quicker. 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. Due to their low health, the shotgun is also very effective against them even when fired from about a dozen feet away.
- 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 99.9% of the time, but be prepared to fire again if for some reason the first shot did not kill him.
- 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 than Shotgun Guys 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: This type requires accurate aim and cool nerves as his shield is impenetrable. Keeping one's distance is strongly advised if weaker weapons like the machinegun are used. The best way to dispatch of this nasty trooper is 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".
- Pistol and Riot Trooper Z-Secs do not appear in Doom 3: Resurrection of Evil.
- The Machine Gun Z-Sec is the most common Z-Sec variant that appears in Resurrection of Evil, while Shotgun Z-Secs are uncommon.