(Reuters) — Two agricultural researchers from China were charged on Friday in connection with a 2013 attempt to steal U.S. rice technology for use in the medical field.

A U.S. grand jury indicted Liu Xuejun, 49, and Sun Yue, 36, for conspiracy to steal trade secrets and conspiracy to commit interstate transportation of stolen property, according to the U.S. Attorney's Office for the Eastern District of Arkansas.

Lawyers for the defendants could not immediately be reached for comment. The Chinese Embassy in Washington also could not immediately be reached.

The charges come as the United States and China have slapped tariffs on imports of billions of dollars' worth of each other's goods in an escalating trade war. The United States alleges that China steals U.S. corporate secrets and wants it to stop doing so.

"Today’s indictment of Liu and Sun should not go unnoticed by those who seek to steal our trade secrets and technology," said Diane Upchurch, special agent in charge of the FBI's field office in Little Rock, Arkansas.

In recent years, U.S. law enforcement officials have urged agriculture executives and security officers to increase their vigilance and report suspicious activity involving farm products, citing a growing economic and national security threat to the sector.

The defendants traveled to the United States to visit rice research and production facilities, including those in Arkansas and Kansas, U.S. officials said. On their way back to China, U.S. customs agents found stolen rice seeds in their luggage, they said.

The seeds were engineered by a company named Ventria Bioscience Inc. to contain certain proteins that can be removed from the rice and used in medicines and pharmaceutical products.

"It is important that this intellectual property be respected and protected from would-be thieves in order for Ventria to sustain itself and continue to thrive as a business," Scott Deeter, Ventria's chief executive officer, told Reuters in an email.

The researchers' trip was organized by two other agricultural scientists from China, Weiqiang Zhang and Wengui Yan, according to prosecutors. They were working in the United States and have previously been convicted in connection with the case and sentenced to prison.

"We remain vigilant in our efforts at protecting both intellectual and real property from theft by other nations," said Cody Hiland, U.S. Attorney for the Eastern District of Arkansas.