A bipartisan and bicameral group of lawmakers sent a letter to President Joe Biden and other administration officials on Wednesday, highlighting concerns that threats from the Iranian government have prevented the U.S. from taking possession of seized Iranian oil.

The letter is focused on the Suez Rajan, a tanker anchored off the coast of Texas. The U.S. seized the Iranian oil onboard the ship in April, in accordance with the U.S.’ sanctions, but the oil onboard has reportedly not yet been off-loaded. The senators link this delay to threats of violence by Iran’s Islamic Revolutionary Guard Corps against any company involved in off-loading the cargo.

“The ability for a designated Foreign Terrorist Organization (FTO) to prevent the transfer of seized assets within U.S. territorial waters is an unprecedented intimidation effort that threatens to undermine our Nation’s security,” the letter reads. “Current public reporting that such an operation cannot be completed due to Iranian threats against U.S. companies and persons undermines the success of this seizure and imperils the U.S. government’s ability to successfully complete future seizures.”

The letter was organized by Sen. Joni Ernst (R-IA) and Rep. Elise Stefanik (R-NY) and signed by Sens. Richard Blumenthal (D-CT), Lindsay Graham (R-SC), Bill Hagerty (R-TN) and Kevin Cramer (R-ND) and Reps. Josh Gottheimer (D-NJ), Jim Banks (R-IN), Don Davis (D-NC) and Max Miller (R-OH).

The letter urges Biden to “work expeditiously to resolve the delay” and continue to enforce the law against FTOs, warning that sanctions enforcement “will become irrelevant if American citizens and companies involved live in fear of Iranian retaliation.”

The lawmakers request a briefing and other information from the administration by Sept. 15 on the status of the Suez Rajan shipment and U.S. oil sanctions against the IRGC, as well as the companies that have refused to assist with the offloading.

The letter also highlights shortfalls in the U.S. Victims of State Sponsored Terrorism Fund, which provides compensation to survivors and others impacted by terror attacks, which is funded in part by seized Iranian assets. According to the lawmakers, funding for the program will be insufficient for 2024, and the program has a $105 billion outstanding balance.

“We owe it to these American families to enforce our sanctions and more aggressively close this gap,” the letter continues. “Permitting the months-long delay in the seizure and transfer of the oil aboard the Suez Rajan, will further delay the federal government’s ability to help make American victims of terrorism whole via USVSST funds. This is unacceptable.”

Ernst, Blumenthal and other senators raised concerns in a letter earlier this year about alleged gaps in enforcement of sanctions on Iranian oil.