Investigators say the marijuana, in loads transported since June, was hidden in pallets of watermelons being trucked from McAllen to a warehouse in Houston.
U.S. Attorney Kenneth Magidson says two men pleaded guilty Wednesday to conspiracy to possess with intent to distribute marijuana. Four others pleaded guilty Friday. The men are from Houston, Roma and Rio Grande City.
All will be sentenced in March. They face up to life in prison and maximum $8 million fines.
Officers confiscated bales of marijuana with a total weight of 3,158 pounds.