REC 2

Dr. Owen (Jonathan Mellor), an official from the Ministry of Health, and a heavily armed GEO team equipped with video cameras are sent into a quarantined apartment building to control the situation. After Martos, a GEO operator, is killed and infected encountering some of the infected, Owen uses religious mantra and rosary to hold him off in a room. It turns out that Owen is actually a priest sent by the Vatican to get a blood sample from a possessed girl named Tristana Medeiros, the same attacker from the end of the previous film.
At the penthouse, Tristana is nowhere to be found. Larra, one of the GEO operators, finds a sample of her blood â the only one that the original priest took from her. Owen performs a religious rite, and the blood combusts, confirming that it is the blood he's looking for; however, Fernandez, commanding officer of the GEO team drops it, rendering it useless. Owen tells them that they must get a blood sample from Tristana herself.
Jennifer's father, who was outside getting medicine for his daughter (the infected Jennifer from the first installment), manages to convince a firefighter to take him into the building. Three teenagers, Tito, Mire and Ori gain access to the building through the sewage system; however, police seal their exit. After the deaths of Larra, Jennifer's father, and the firefighter from attacks by the possessed, the two groups find each other. However, Tito has been bitten, and they restrain him and lock Mire and Ori in a room where their fates are left unknown. The group then finds Ãngela Vidal (Manuela Velasco), the reporter from REC, who still has her camera with her. Owen forces Tito to tell him where Tristana Medeiros is. The demon possessing Tito tells him that she is "in the highest." Owen figures out that this means that she is in the penthouse. The demon then gives a hint that the "light" blinds them from seeing the path. When Ãngela is asked how she saw Tristana, she says she saw her through the camera's night vision. Owen deduces that some things can only be seen in the dark.
They go back to the penthouse. When they turn on the camera's night vision, a door appears that leads them to a room. They find a tub filled with water that cannot be seen when the lights are on. While the GEO chief, Fernandez investigates it, Tristana leaps out and drags him in and then disappears as the light is turned on. When Tristana attacks them again, Ãngela blows her head off with a shotgun, finally killing her. Owen is enraged because he needed the blood sample, but all Ãngela wants to do is leave the building. At this point, the only survivors are Ãngela, Owen, and Rosso, the GEO cameraman. When Owen refuses to authorize their exit, Ãngela starts to beat him. Rosso tells her to stop, but she shoots and kills him. It is revealed that the demon possessing Tristana has now possessed Ãngela so she can leave the building. After telling Owen that she does not need him to get out, as she can impersonate his voice, she kills him. She takes his radio and says that the mission is over and they can let them out. She also tells them that he is staying behind because he is infected and the only survivor is a woman, Ãngela. When asked how the woman survived, the demonic Ãngela looks at the camera and smiles.
The ending of the first film is shown, but extended to reveal how the demon possessed Ãngela (by forcing her mouth open and transferring some type of large parasite into her mouth), and then as Ãngela, went into hiding as Owen and the GEO team broke into the penthouse earlier in the film.

Question: How did the teenagers gain access to the building?
Answer: sewage system