U.S. solar panel imports plunged in May as foreign manufacturers moved to limit their exposure to the American market amid a federal trade probe.

The number of shipping containers delivering solar panels to U.S. ports was down 20% from the prior month and 46% from a year earlier, according to research company Panjiva. Imports declined after the U.S. Commerce Department opened an investigation in March into whether manufacturers moved some operations to Southeast Asia in recent years to skirt tariffs on China.

The Biden administration announced executive actions June 6 to boost domestic panel supplies as the investigation continues, including suspending potential tariffs on Vietnam, Malaysia, Thailand and Cambodia for two years. The four countries are the focus of the Commerce Department investigation. They accounted for about 85% of U.S. solar panel imports in the first quarter.

The Biden administration's decision to intervene is expected to help the U.S. solar market move forward this year with some projects that had been jeopardized by limited panel supplies and rising prices, according to the Solar Energy Industries Association, or SEIA, a lobbying group.

Because the industry knows that new tariffs could still be imposed, the reprieve could generate "more activity than probably we even originally anticipated, which is almost insane to say," José Mas, CEO of MasTec Inc., an infrastructure company, said at an investor conference June 9.

However, Mas added that it is unclear how quickly manufacturers in Southeast Asia can ramp up factories that have been idled since the U.S. government announced its investigation.

"So, by the time that they're able to gear back up, by the time that they're able to ship [product] with all of the supply chain issues that exist, when does it get to the U.S.? And when does it actually ramp meaningfully? We don't know," Mas said. "Now, how many developers also will start projects without the panels being there because they know it's coming? That's another question."

Longer term, delaying the threat of tariffs gives the solar industry time to move manufacturing to countries that are not under investigation by the Commerce Department, said Jayshree Desai, chief corporate development officer at Quanta Services Inc., another infrastructure company, at the investor conference June 8.

"[The] solar industry has been through these times before, different types of tariffs, different types of policy shocks," Desai said. "And it's always adjusted."

Panjiva is the supply chain research unit of S&P Global Market Intelligence, a division of S&P Global Inc.

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