Fast & Furious

Five years after the first film, Dominic Toretto and his new crew, consisting of his girlfriend Letty, Tego Leo, Rico Santos,Cara Mirtha and Han Seoul-Oh, are hijacking fuel tankers in the Dominican Republic. Dominic suspects that the police are on their trail, forcing the crew to disband and go their separate ways, with Han deciding to return to Tokyo. Realizing that he must leave, he runs, leaving Letty behind to protect her from harm.
Three months later, Dominic is now residing in Panama City. He gets a call from his sister, Mia Toretto, who tells him that Letty has been murdered. Dominic heads back to Los Angeles to attend her funeral and examine the crash and finds traces of nitromethane on the ground. He visits the only car mechanic that sells nitro-methane in LA and forces him into giving him the name David Park, the man who ordered the fuel, and informs him that the only car that uses nitro-methane in the area is a green 1972 Ford Gran Torino Sport.
Meanwhile, F.B.I. agent Brian O'Conner is trying to track down a drug lord, Arturo Braga. His search leads him to David Park, and he tracks him down using an illegal modification record on his car. Dominic arrives at Park's apartment first and hangs him out of the window by his ankles before Brian arrives. Brian saves Park and Park becomes the F.B.I.'s new informant. Park gets Brian into a street race. Brian selects a modified 2002 Nissan Skyline GT-R R34[5] from the impound lot. Dominic races in his modified Chevelle. Gisele Yashar, the liaison for Braga, reveals that the winner will become the last driver on a team that traffics heroin between the MexicoâUnited States border. Dominic wins by bumping Brian's car while it is in nitro, making him lose control. Brian uses his power as an F.B.I. agent to arrest another driver, Dwight Mueller, and takes his place on the team.
The team meets one of Braga's men, Fenix, and Dominic notices that Fenix drives the same Torino the mechanic described. They drive across the border using underground tunnels to avoid detection. Brian has prior knowledge that, after the heroin was delivered, Braga ordered the drivers to be killed. Fenix killed Letty personally, and after a stand-off, Dominic detonates his car with nitrous to distract Braga's men, and Brian hijacks a 1999 Hummer H1 with $60 million worth of heroin in it. Dominic and Brian drive back to L.A. and hide the heroin in a police impound lot, where Brian picks up a modified 2008 Subaru Impreza WRX STI Hatchback, and they drive to Dominic's hideout.
Dominic finds out Brian was the last person to contact Letty, which results in Dominic attacking Brian until Brian explains that Letty was working undercover, tracking down Braga in exchange for clearing Dominic's record. Brian tells his superiors that in exchange for Dominic's pardon, he will lure Braga into a trap, forcing him to show up to exchange money for the heroin. At the drop site, the man who claims to be "Braga", is revealed as a decoy, and "Campos", the real Braga, escapes and flees to Mexico.
Brian and Dominic travel to Mexico to catch Braga. They find him at a church and apprehend him. As Braga's henchmen try to rescue him, Brian and Dominic drive through the underground tunnels back to the United States. Brian crashes his car after taking fire from Braga's men. He is then injured after being T-boned by Fenix. Before Fenix can kill Brian, Dominic drives into and kills Fenix. As police and helicopters approach the crash site on the American side of the border, Brian tells Dominic to leave, but Dominic refuses, saying he is tired of running. The judge sentences Dominic to 25 years to life. Dominic boards a prison bus that will transport him to Lompoc penitentiary. As the bus drives down the road, Brian, Mia, Leo and Santos arrive in their cars to intercept it.

Question: Why do Brian and Dominic travel to Mexico?
Answer: To catch Braga.