The U.S. maritime regulator will investigate whether shipping lines are impairing the ability of American farmers to reach foreign markets by holding back empty containers needed to export goods.

The Federal Maritime Commission said in a statement Friday that it would investigate actions by ocean carriers at the ports of Los Angeles, Long Beach and New York, as a crush of imports has triggered bottlenecks at the country’s biggest export gateways and buffeted inbound and outbound supply chains.

U.S. exporters of soybeans, cotton, lumber and hay say they can’t find the boxes they need at distribution points in the middle of the country because shipping companies are rushing to unpack containers coming in from Asia and send them back empty, rather than loading them with American exports.

High demand by retailers and manufacturers to replenish inventories that had been depleted during the pandemic has sent shipping rates out of Asia soaring, and ocean carriers can command up to eight times more for Asia exports than suppliers of lower-value U.S. agricultural goods will pay.