This is a main storyline mission involving Larissa. Apparently a man named Lopez has some information in his car. We need to steal it - the car is parked outside the strip club where Larissa works. Get in your car and head there.
It turns out Lopez is a policeman! As a result, you will be stealing a police car and, as you would expect, this gets you a large wanted level straight away. You need to lose your wanted level, so drive around collecting police badges, or find a secluded spot to hide.
Once you've lost the wanted level, return the car to Larissa. It turns out someone stole a detonator and plastic explosives from a police evidence locker just a few days before the explosion happened. So the police may indeed have carried out the hit, or a policeman may have sold the items to someone else.
Reward: 1,500R$