Five tons of school supplies from the Lawrence-based Monarch Watch have hit a roadblock on their annual migration south to Mexico.

Monarch Watch officials struggled to raise enough money to deliver the donated school supplies. They finally collected the money, only to have Mexican customs officials turn them around at the border.

There were so many school supplies that border authorities thought the semitrailer’s contents were intended to be sold, not donated.

“We were so successful getting materials together we attracted the attention of the people in Mexico,” said Orley “Chip” Taylor, the organization’s director.

Taylor said Monarch Watch had been negotiating with Mexican authorities to allow delivery to Ocampo, a town near the site where monarch butterflies spend their winters. Taylor said he’d been assured government approval and delivery would be this week.

Meanwhile, the supplies — donated from across the United States — have been in a warehouse in McAllen, Texas, on the U.S.-Mexico border, for three weeks.

In past years, Taylor said, border officials let the school supply truck through, or he paid a small amount for duty for the supplies, because the truck carried a smaller amount of materials. He said the Mexican government had cracked down on donations being delivered in the country because some were then sold without paying proper taxes.

Taylor said a teacher in Ocampo had agreed to distribute the materials to about 30 to 60 schools once they make it into the country.

“It looks like the whole thing will work out,” he said. “It’s a little more piecemeal than usual because of the difficulties getting things across the border.”

The Adopt-a-Classroom trip is usually in January. But Monarch Watch had raised only about $4,000 of the $12,000 it takes to make the delivery by then, forcing consideration of canceling the delivery.

Taylor said a plea for additional money brought in $7,000 more, enough to complete the trip in March, when he and fellow researchers usually drive to Mexico to observe the butterfly population there.

Taylor said he’s already had discussions with the mayor of Ocampo about how to ship the supplies next year. He said Monarch Watch may transfer the materials to the state or U.S. government for transport across the border.

“We’ve never encountered this kind of problem before,” he said. “At least we know it will get there, and the kids will enjoy the stuff that’s delivered.”