The Iranian-flagged container ship seized by U.S. forces in the Arabian Sea on Sunday was sanctioned by the Treasury Department in 2020, during President Trump’s first term in office. Treasury officials said at the time that they were targeting the 960-foot-long vessel, the Touska, over links to Iran’s financial entities and weapons programs.

“The TOUSKA is under U.S. Treasury Sanctions because of their prior history of illegal activity,” Mr. Trump wrote on social media on Sunday. “We have full custody of the ship, and are seeing what’s on board!”

Mr. Trump said the ship had tried to evade the U.S. blockade on Iranian ports that went into effect last week. A U.S. Navy destroyer repeatedly warned the ship to stop before firing on the engine room and disabling it, U.S. Central Command said. A U.S. military official said Monday that a boarding team of Marines was searching up to 5,000 containers aboard the Touska.

Iran’s military condemned the seizure as an act of “armed piracy” and vowed to retaliate, but said it was waiting to protect the ship’s crew members and some of their family members.

Data shared by MarineTraffic, a website that tracks global shipping, indicated that the Touska had departed Zhuhai, China, on March 30, after docking for almost a day. The Touska had previously signaled its location near Shanghai. Port logs show the vessel then arrived around 12 days later in Port Klang in Malaysia, before traveling toward the Persian Gulf. According to Central Command, the ship was heading to the port of Bandar Abbas in southern Iran.

MarineTraffic data from Sunday shows the vessel turning abruptly and appearing to drift.

The origin of the ship’s cargo was not immediately clear. Central Command did not immediately respond to requests for information about the cargo or the identities of the crew members.

Since the U.S.-Israeli war with Iran began on Feb. 28, U.S. intelligence officials have obtained information that China may have sent a shipment of weapons to Iran, according to U.S. officials. A spokesman for the Chinese Embassy in the United States strongly denied his government had shipped missiles to Iran during the war.

Adina Renner contributed reporting.