Since when Weiss killed two policemen viz. Sandra Bates and Theo Nelson, he was on the suspect list of Brant. Brant knew about him from Radnor, who gave him the news of Weiss's killing a police dog and posting it on Youtube. Since then Brant was suspecting him. Also Brant recognized Weiss in his apartment due to his fight with him about a year ago. So whenever any killing took place he tried to connect it with Weiss. Eventually from his criminal records in the police station with the help of a lady police,, Brant discovered that any police who arrested Weiss, Weiss is attacking them particularly and Falls was the next. So assuming it is true, Brant took no risk and sent immediate help to her apartment and we find Weiss falls flat in his attempt to murder Halls.