PHOENIX — Local and federal law enforcement officers said they have broken up a drug transportation ring that was bringing 60,000 pounds of marijuana into Arizona literally one backpack at a time.

Pinal County Sheriff Paul Babeu said the operation came to light with a routine traffic stop last year that yielded about $229,000 in cash. But Babeu said that, rather than simply take down those in the vehicle, he worked with the Drug Enforcement Agency to go after the whole operation.

What they found, the sheriff said, was a group that hand carried marijuana across the border through the Tohono O’odham Reservation.

Smugglers, carrying 50 to 100 pounds apiece, could take up to 10 days to reach their destination, with the organization leaving food and water at some predetermined locations. Eventually the drugs would wind up at “stash’’ houses in Pinal County.

To date, 21 people have been arrested, including Robert Hernandez, 38, of Arizona City, who Goddard said is the alleged leader of the organization. Charges against him include transporting marijuana, conspiracy, money laundering and use of a minor to commit a drug offense, that last one stemming from hiring teens to take the drug from the individual backpackers and drive it the last length to the stash houses.

They also have seized nearly 4,000 pounds of marijuana, 21 vehicles and several assault rifles.

Babeu said it is not surprising that the smugglers chose to route their traffic through the reservation, which straddles both sides of the international border. He said while tribal police helped, the sheer size of the reservation and the sparse population make that a popular route.

“These people don’t want to be detected,’’ the sheriff said.

More to the point, Babeu continued, much of the border through the reservation is without any sort of meaningful barrier.

“The criminal element exploits the scene,’’ he said.

Attorney General Terry Goddard said it made sense to pursue the investigation, rather than just go for some quick publicity.

“So what we have is not just one or two small fry that are arrested in the desert, they stumbled in the wrong location,’’ he said.

“We’re talking about an entire organization which was identified through some very good police work over a period of about nine months,’’ Goddard continued. “And then, through a multi-agency cooperation, we’re bringing them to an end. They’re gone.’’

Goddard said he believes this particular group was operating for about three years. But he conceded that even taking out a group that was bringing in 60,000 pounds a year amounts to just a portion of what is coming into Arizona.

“I’d love to tell you that we’ve struck a death blow to the whole organization,’’ he said. “I cannot say that.’’

Nor can he say which for Mexican drug cartel this particular transportation group was working.

Goddard said, though, his office and the police agencies now have “a much higher level of understanding of what it is and how they do it.’’ And he said that has given law enforcement some clues on where to find them.