We helped you track down Penguin. Now it's time to find out if Black Mask is no more.

Solve the Crime Scene Inside Lacey Towers. 

According to Alfred, Black Mask has been murdered. Proceed to Lacey Towers via the Batwing. Locate the apartment south from you. Dismiss the policemen, then use the Cryptographic Sequencer to hack and decode the system. Now head over to the balcony, defeat the guards and enter Lacey Towers. 

At Sionis' Safehouse, equip the Evidence Scanner and investigate the scene. You need to scan both corpses and the colorations in the floor, and then replay the scene to get a better understanding. Repeat this process and follow the red marker – this leads to a fabric substance on the shooter. Now scan the kitchen for another hint. Replay this scene and find a cell phone. The evidence points to The Joker.

Acquire the Concussion Detonator at the Batcave

You now have a new objective: Acquire the Concussion Detonator at the Batcave. This is a rather simple task – return to your base and snag the item from Batman's workshop from the lower level of the cave. Be sure to talk to Alfred for additional XP upon your return.  

The Concussion Detonator dazes and blinds enemies upon impact without the need of a nasty explosion. Even better, its impact can cause foes to physically harm each other. Hold LT/L1 to aim the device, and RT/R1 to throw and detonate it. Try deploying this bad boy when dealing with a tough crowd.  

Access the GCPD Servers/Gain Access to the GCPD via the Rooftop 

Batman needs to get more Intel on the murder case;  more importantly, info on The Joker. The police department is located in the southeast section of Gotham (just south of Burnley). Swing your way there. The station is heavily guarded. Use high ground and dismiss each guard individually. You gain entry to the Rooftop Maintenance from the roof's door.

To reach the GCPD Bullpen area, deploy Explosive Gel and ride the elevator down. Use the air duct to gain entry to the SWAT Manager's Office. The officers in the building are easy takedowns.  The Conference room can be accessed through the vent via the Batclaw. Carry on and avoid being spotted by using the Remote Claw.  A password at the Reception's panel can be encrypted: "Serve Protect" is decoded using the Cryptographic Sequencer. 

Make your way to the Interrogation Room to the north and interrogate Penguin's arm dealer again, known as Ricky "Loose Lips." A boarded-window can be blown up with your Explosive Gel, allowing access to a route to the GCPD Bullpen on higher ground.  It's crucial to use the floor shafts to silently defeat the officers here; even better, deploy zip-lines with the Remote Claw to get around with ease.  Once you've cleared the area, you'll discover a security panel at the door – hack into it with the Cryptographic Sequencer revealing Peace Officer as the passcode. At the Server Room, the panel is inaccessible for hacking, and you get a new mission. 

Acquire the Disruptor At the Evidence Locker 

Proceed to the Basement Access nearby. Two officers guard it. Access the code panel at the elevator and decode it to reveal Gotham Defenders as the password. The passage leads you to the Holding Cells. Eliminate the guards best by using the Fire Extinguishers for distraction. Afterward, hit the switch on the western gate and go to the Infirmary. Blowing up the wall in the Infirmary using Explosive Gel leads to the Derelict Elevator Shaft: Look above and use the Remote Claw to form a zip-line. Swing up like a bat! 

Go through the ventilating path to reach the Evidence Locker hall. You discover the Disruptor on the desk inside the cell.  The Disruptor can dismantle and bypass electronics such as jammers and weapons.  Aim with LT/L1 and hold RT/R1 to deploy the device. Now return to the Holding Cells on the lower level. It appears Batman is trapped! Equip the Disruptor and target the gray box above the door panel to unjam it. Then hack into it with the Cryptographic Sequencer. The Code is Lockdown, which unlocks the gate. 

Unfortunately, the prison's inmates are now roaming free in the hall. It should be noted you encounter a "Mini-Bane" here – some of his attacks cannot be evaded. Instead,  pounce over them when they try to grab you and shake them with your spin attack.  After, head back to the Server Room hall by ascending the balcony above. Back at the Server Room's doorway,  disrupt the jammer with the Disruptor and then break the code at the panel with the Cryptographic Sequencer to reveal the password Authorized Only.  Head inside and chat with Barbara Gordon. She informs you on important criminal data and then mentions to proceed to the sewers located beneath the police department.  

Escape the GCPD

The Police aren't too friendly to Batman and crash his chat with Barbara. Escape by going through the vent, which leads to the Shooting Range. Tackle the cops that ambush you – it's a good time to test out that Disruptor and disable their guns here. Otherwise, dismantle them quickly with your most-used tactics. Moving on, hack the door panel beyond the fence; the code is "Lock And Load."  You gain entry to the Investigations corridor. Proceed north into the Bullpen. Batman takes the elevator up.  

A cinema ensues and Batman encounters Commissioner Gordon in the Rooftop Maintenance area. Officers woefully ruin the meeting; throw smoke to confuse them and put them down. Afterwards, exit via the door leading to the rooftop. Assassinate the two clueless guards at the helipad easily and you gain a new quest: Access the Sewers Beneath GCPD.  

Note: Upon completing this objective, Barbara Gordon calls and requests to locate  six hidden arms caches. This is an optional side-quest that we will cover later. 

Access the Sewers Beneath GCPD

The sewers entry is just northeast from your position. Check your map for its location. You need to venture through the alley on ground level to get there. 

If you enter from the east entrance, you deal with cops. Entering from the south avoids them. Either way, open the sewer vault in the alleyway. Time to get icky! 

Uplink the National Criminal Database to the Batcomputer

Black Mask's henchmen guard the first section of the sewer. Additionally, two new enemies known as Martial Artists are present. They're quick and have a three-hit combo that you may need to evade. Ambush them using a Glide Kick, counter when necessary and clean house. When defeated, approach the wooden boards covering the sewer gate and blast it open with some Explosive Gel. The passage leads to the Water Treatment area. Work your upward, be careful of the steam pipes and eliminate Black Mask's men on the upper catwalk.  Now shimmy across the outer walls to reach higher ground; you'll find a control panel on the upper dock. Hit the switch to stop the sewer's water flow. 

Switch to Detective Vision and spot the highlighted-valve on the pipeline. Grapple and pull it open with your Batclaw. This shifts the steam's direction, allowing you to pass. Lastly, create a zip-line above with the Remote Claw from the marker on the wall across from you.  Ride the cable line across and crash through the window – and the guard. Find a panel on the outside catwalk you can hack. The passcode is Pump Access.  This opens a passage on the gate below you. Glide there easily with a Glide Kick, taking down the guard in one hit.  

Follow the passage to reach the GCPD Telecom Room. Black Mask's men patrol the following zone on a lift. Descend there and dismiss them. Then hit the button and ride up the elevator to the control room. Prepare for a moderately challenging fight with Black Mask's men, two of which are a Martial Artist and Mini-Bane. After, access the computer console: the Intel shows The Joker was behind the murder at Lacey Towers.