It's not just the number of people coming into the U.S. that is a concern for the Department of Homeland Security, but it's from which countries they are coming.They cross in the cover of darkness and in broad daylight.Border Patrol agents in San Diego stop nearly 400 illegal border crossers each day."We have five or six a day; that's just on a day shift," said Border Patrol agent Tim Feige.There is no telling how many they dont stop."We never know what they're here for or what their intentions are," added Feige.10News joined agents on patrol and saw firsthand what they face. In one incident, two men and one woman tried to sneak by right in front of agents. They first hid and then surrendered. The group turned out to be Mexican citizens with no criminal records, and they were processed and sent back to Mexico.Because 85 percent of those apprehended by agents are from Mexico, the Department of Homeland Security classifies the others detained as "OTMs," or Other Than Mexican."They try to pass themselves off as being from Mexico," said Border Patrol agent Allen Gustafson.Last year, OTMs came from 148 of the 193 countries in the world. Several came from what Homeland Security terms special interest countries -- countries that are considered a great threat.10News learned that in the last six months, agents along the Southwest border caught 15 people from Iran, 35 from Pakistan, 12 from Jordan, two from Syria and five from Lebanon. These are numbers Homeland Security would not officially release."We're more aware, not only of terrorists, but terrorist weapons," said Gustafson.Agents who patrol the coastline have radiation detection equipment and try to at least eyeball every incoming boat."The busiest time is the fishing months, when there's a lot of boat traffic. Everyone has got a boat out here; they try to blend in with the regular traffic," said Gustafson.Potential terrorists are not the only concern.Agents said many violent criminals cross the border."In fact, we caught a person who was number 17 on Mexico's most wanted list," said Feige.A top priority for the Federal Bureau of Investigation is to stop the influx of a notoriously brutal gang called the MS-13 -- the Mara Salvatrucha -- a group linked to violence across California and 32 other U.S. states.According to reports, El Salvador, Guatemala and Honduras export members of the MS-13 gang.10News learned that agents have stopped 26,035 undocumented people from El Salvador, 11,781 from Guatemala and 16,370 from Honduras in the last six months.The two fences that line the U.S.-Mexico border stop car traffic, but agents said they look to slow down the people on foot."If we have a group jumping the fence, we can get there twice as fast as maybe one of the bigger trucks can," said Border Patrol supervisor George Gibson.The goal of agents is to catch those crossing and those who help the crossers."They usually use these ladders they weld out of rebar, so one of our objectives is to try and grab that ladder before they get it back south," said Gibson.It is rewarding but frustrating work. The stakes are high, and every day it is more of the same.In the last six months, nearly 1,200 people from China were caught trying to enter the U.S. illegally.Agents said Chinese nationals pay smugglers up to $30,000 for passage to the U.S.