Border patrol agents arrested two California-based Marines for helping to smuggle undocumented immigrants across the southern border into San Diego County, officials said Tuesday.

Marine Lance Cpls. Byron Law II and David Salazar-Quintero, from Camp Pendleton, were taken into custody July 3 and charged with committing "transportation of certain aliens for financial gain."

Each said the other was responsible for getting him into the business of helping smuggle immigrants, according to a federal complaint.

A border patrol agent spotted a black car pulling off an interstate, on to a dirt turnaround, before quickly getting back on to the highway. That agent believed "this could be a vehicle looking to pick up" undocumented immigrants, court documents said.

That pickup point is about 20 miles east of a U.S. port of entry at Tecate and seven miles north of the border, officials said. The location is "where undocumented immigrants often wait to be picked up," Customs and Border Protection spokesman Ralph DeSio told NBC News on Tuesday.

A short time after that initial sighting, another agent pulled over the black car, carrying Law, Salazar-Quintero and three others who "stated they are citizens of Mexico without any immigration documents that would allow them to enter or remain in the United States legally," according to the complaint.

Two of the passengers "stated they were going to pay $8,000 to be smuggled into the United States" with "destinations to New Jersey and Los Angeles," court documents said.

Law and Salazar-Quintero both waived their Miranda rights and told investigators they had done this work before.

"Salazar stated he was originally introduced to smuggling by Byron Darnell Law II who introduced him to a man that recruited him," according to the complaint.

Law said they had done a similar job a day before, on July 2, when they picked up a passenger, also off an interstate in eastern San Diego County, and took him to a meetup point.

Law stated that "Salazar called and asked him if he was willing to make $1,000 USD picking up an illegal alien," the complaint said. "Law stated they picked up a single illegal alien at the Jacumba Exit on the eastbound lanes of Interstate 8 and transported him to a McDonald's parking lot in Del Mar. They met an individual who arrived in a blue Nissan Murano."

A Camp Pendleton representative told NBC San Diego that its command is "aware of the charges facing Lance Cpl. Law and Lance Cpl. Salazar-Quintero," and will "continue to cooperate fully with the investigative efforts into this matter."

A lawyer for Lance Cpl. Salazar-Quintero declined to discuss the case when reached by NBC News. An attorney for Law could not be immediately reached for comment on Tuesday.