Go to the church where Tyler's gang is guarding the chemicals. Ask about what's going on: you will need a Speech rating of 70% or more in order to progress the quest (having less than 70% only lets you give Tyler a casual greeting without consequences). Keep talking to him and eventually he will reveal that they are throwing a party.
When you return to Lara, you will find that you cannot collect your reward from her. Instead, she will demand that you help her attack the gang.