Borderline

The film follows a United States Border Patrol Agent in Charge, Jeb Maynard (Bronson), who is forced to track down the killers of a young Mexican boy and his colleague and friend, a veteran Senior Patrol Agent, "Scooter" Jackson, portrayed by (Wilford Brimley). Jeb Maynard is the Patrol Agent in Charge of the "fictional" Otay Border Patrol Station, located in the hills east of San Diego, CA. Otay Station is a composite of the actual El Cajon, CA and Brown Field, CA Border Patrol Stations. He is helped by the young boy's mother, Elena Morales, (Karmin Murcelo) and a rookie Border Patrol Agent, Jimmy Fantes, (Kirby).Senior Patrol Agent Jackson and the young boy are murdered by Hotchkiss, a ruthless alien smuggler, also called "the Marine" by the aliens, portrayed by (Ed Harris). The murders take place when a truckload of illegal aliens being smuggled by Hotchkiss is stopped by Senior Patrol Agent Jackson while on routine road patrol. The truck has boxes of tomatoes on top of a hidden roof on the back of the truck. Under the roof is a hidden compartment containing the load of illegal aliens. Hotchkiss shoots "Scooter" from a concealed position in the back of the truck with a sawed-off shotgun at close range. The Mexican boy is badly wounded by stray buck shot so Hotchkiss finishes him off with another blast from his shotgun. Hotchkiss drags the bodies into the bushes along the side of the road and conceals the Border Patrol sedan in the same bushes. The other alien smuggler driving the truck with Hotchkiss becomes very nervous about the murders so Hotchkiss later kills him to keep him silent after they drop off the load of illegal aliens at a local fruit ranch, owned by well to do fruit farmer Carl Richards, and used as a front to smuggle aliens. Hotchkiss later abandons the truck used to smuggle the aliens along a rural road. Hotchkiss leaves the body of the other alien smuggler who was driving the truck the night of the murders, along with the truck, and conceals some small bags of marijuana in the truck to make it appear to be a drug smuggler's vehicle. Other Border Patrol Agents find the bodies of Agent "Scooter" Jackson and the boy, and the Border Patrol car later in the morning. Agent Fantes finds some fresh tomatoes near the bodies. Agent Maynard notices some boot prints in the dirt among all the other foot prints at the crime scene. These particular prints were made by a pair of military style boots with some odd markings in the soles. One of a veteran Border Patrol Agent's professional skills is "sign-cutting." "Sign-cutting" is the skill to examine, analyze, and interpret tracks and marks made in the ground. Patrol Agents use this skill to track groups of illegal aliens crossing the border. Maynard has Fantes take the tomatoes to the Agriculture department of a nearby university for analysis. The F.B.I. is called to investigate. The F.B.I. has primary juridiction investigating the murder of any federal agents. A couple of days later the abandoned truck is found with the drugs. The F.B.I. agents investigating the murders wrongly conclude that Agent "Scooter" Jackson stopped some drug smugglers that night and was shot because of it. Hotchkiss is running a sophisticated and highly profitable alien smuggling operation between Mexico and the United States. Maynard and Fantes start checking the trails in the hills that the smugglers use to bring in illegal aliens and drugs. They find the same military boot prints along a trail where a Border Patrol electronic ground sensor has been dug up and disabled. Hotchkiss is a former U.S. Marine who had been trained on such equipment while in the Marine Corps. He discovered the sensor and disabled it on a prior smuggling run.Jeb Maynard thinks that "Scooter" Jackson was murdered by alien smugglers and tries to convince the F.B.I. that the marijuana in the truck was merely a ruse. The F.B.I. does not believe him. After "Scooter" Jackson's funeral with full honors, Jeb tells his boss, I&NS; Commissioner Malcolm Wallace, about his suspicions. Commissioner Wallace cautions Jeb to proceed carefully in the matter. So Jeb begins his own investigation of the murders with the assistance of Patrol Agent Jimmy Fantes. Maynard had found a piece of paper with a San Diego address on it in the murdered Mexican boy's clothes. Maynard goes to the address and finds the boy's mother, Elena Morales, working at a well to do family's home as their nanny. Maynard takes Elena to the morgue so she can identify her son's remains. Maynard then asks for her help in finding her son's killers. Elena is brave and she agrees. Maynard goes undercover posing as the woman's cousin. They cross into Mexico and she introduces him to an alien smuggler in Tijuana, Mexico who brought her over the border when she last crossed. This smuggler is Hotchkiss's partner in Mexico. Maynard's physical features are such that he can pass for Mexican. Elena tells the smuggler that her cousin is simple minded and doesn't talk much. Jeb Maynard cannot speak Spanish well enough to pass as a native Mexican. Elena pays the smuggler with money Jeb gave her.They are smuggled across the U.S.-Mexican border and through the hills east of San Diego into the United States along with a group of thirty illegal aliens. Maynard hears the smuggler guiding the group talking about "the Marine" who runs things" to Elena. But then the group is ambushed by bandits who want to rob the group. Jeb and Elena escape unharmed. Jeb and Elena walk back towards the suburbs of San Diego. Jeb gets Elena home and thanks her for her help. He tell Elena that she should stop by his office the following week and he will try to help her straighten out her immigration status in the United States.Maynard gets home, cleans up, and tries to get some sleep. Jimmy Fantes stops by Maynard's home to report to him that the agriculture report came back on the tomatoes found at the crime scene. These particular tomatoes were treated with a particular new brand of pesticide. Fantes checked with the local office of the U.S. Department of Agriculture and only a few large farms in the local area use that brand of pesticide. Maynard and Fantes meet later at the Border Patrol station to plan their next move. They put on their Border Patrol uniforms and drive in Jeb's Border Patrol S.U.V. to check the local fruit farms that use that pesticide. They finally end up at Carl Richards's large fruit ranch. Jeb knows of Richards's reputation for hiring illegal aliens for poor wages to harvest his fruit. Hotchkiss is in the main ranch house when Maynard and Fantes arrive. Hotchkiss stays inside the house and Richards goes outside to speak to the Border Patrol Agents. While talking to Richards, Jeb Maynard notices the same style military boot prints in the dirt near the main house that he detected at the crime scene.Jeb Maynard and Jim Fantes set up a surveillance of the Richards Ranch with the assistance of Border Patrol Agents Lambert and Monroe. While using binoculars they see Hotchkiss wearing his combat boots and camouflage marine field jacket departing the house with other smugglers, including the Mexican smuggler who was guiding the group that Jeb and Elena had infiltrated. Maynard has found "the Marine". Hotchkiss is planning a large alien smuggling run soon that will bring hundreds of illegal aliens into the U.S. in one evening. Then he plans on shutting down operations for a while until things cool off.From what he witnesses on the surveillance of the ranch over the following days Jeb Maynard deduces Hotchkiss's plans. Maynard get all his agents together at the station and plans an operation for the following evening. The Border Patrol is going to raid the Richards Ranch. Jeb Maynard and his Border Patrol Agents raid the ranch in the late evening/early morning capturing all the illegal aliens and the smugglers bringing them into the ranch's main barn. At dawn Hotchkiss arrives at the ranch with the last load of illegal aliens. As the Patrol Agents attempt to arrest him Hotchkiss pulls out a model MAC-10 nine millimeter machine pistol and fires a burst at the agents. The agents take cover and Hotchkiss jumps in a car and speeds away. Jeb Maynard pursues him in a Border Patrol four wheel drive truck. Hotchkiss attempts to lose Maynard on an old dirt road. The road dead ends and Hotchkiss runs on foot into the surrounding trees and bushes. Maynard draws his Smith and Wesson Model 28 .357 Magnum revolver from its holster and starts tracking Hotchkiss through the forest. Hotchkiss cirlces around back to the vehicles thinking he has given Maynard the slip. Just as Hotchkiss is about to get back in his car Maynard emerges from the treeline with his revolver pointed at Hotchkiss and says, "the end of the road." Hotchkiss wheels about and starts firing his MAC-10 machine pistol at Maynard. Maynard is a better shot and kills Hotchkiss with a single shot from his .357 Magnum.The film ends with Maynard and Fantes watching Richards leaving the United States Court House in San Diego, CA after he has been sentenced to three years in federal prison for alien smuggling. Fantes laments that Richards will probably go right back to smuggling aliens within a month of his release from prison. Jeb Maynard responds by saying, It's okay kid, we'll be there waiting for him.

Question: why were  Maynard and Fantes checking the hills?
Answer: Because they were smuggling illegal aliens.