Today's D Brief: Tanker hijacked; More SecDef scrutiny; Demining tech; Time to close Gitmo?; And a bit more.

Iran’s navy hijacked a tanker ship carrying Iraqi oil in the Gulf of Oman on Thursday. The Marshall Islands-flagged vessel is known as the St Nikolas, and was bound for Turkey when it was captured by Iranian forces just south of the United Arab Emirates, according to Iranian state-run Press TV.  

At least four men “wearing military style black uniforms with black masks” boarded the ship early Thursday morning while it was traveling about 50 nautical miles east of Oman, British authorities said. The ship’s crew consists of 18 Filipino nationals and one Greek national, according to the Associated Press

Rewind: U.S. authorities captured the very same ship last April off the Gulf Coast for carrying almost a million barrels of sanctioned Iranian crude oil, which was allegedly going to be sold to China; that oil was later offloaded in Houston. The vessel was known as the Suez Rajan when it was seized by U.S. officials; its owners changed its name after the incident. 

Big picture: This action had been expected, as historian Sal Mercogliano pointed out on social media Thursday. Sal also has a great video on YouTube explaining what he’s witnessed from tracking software monitoring the Red Sea—including a convincing illustration of an Iranian vessel (Behshad) that seems to be feeding the Houthis with targeting information. 

Worth noting: Many Chinese carriers and container ships are traveling the Red Sea without incident, as industry-watcher Lloyd’s List reported Tuesday. And some Chinese vessels, like Cosco’s Shengshi, are making clear that they are a “CHINESE COMPANY” in the free text portion of the automatic identification system software used by ship trackers across the web. 

Related reading: 

Welcome to this Thursday edition of The D Brief, brought to you by Ben Watson with Bradley Peniston. Share your newsletter tips, reading recommendations, or feedback for the year ahead here. And if you’re not already subscribed, you can do that here. On this day in 1863, the U.S. military defeated a contingent of treasonous Confederates during the three-day Battle of Fort Hindman at the mouth of the Arkansas River. This victory paved the way for Ulysses S. Grant's legendary Vicksburg campaign, which eventually concluded on July 4 about 150 miles south of Fort Hindman.

Republican senators demand answers about Defense Secretary Lloyd Austin's recent hospitalization for prostate cancer, promising in a statement Thursday to investigate the military's “failure to follow the law regarding chain of command” when Austin was under anesthesia at the Walter Reed National Military Medical Center in late December and early January. 

All GOP senators on the Armed Services Committee submitted 17 questions for Austin in a letter published Wednesday. Queries include “the exact time Secretary Austin became incapacitated,” whether Austin or his staff made a decision not to inform Congress, when the White House was notified, whether or not “all aspects of the nuclear enterprise” were transferred to Austin’s deputy at any point, if the National Airborne Operations Center was dispatched, exactly what decisions Deputy Defense Secretary Kathleen Hicks made as acting Pentagon chief, a full list of Hicks’ traveling staff, and more. 

“Either Secretary Hicks did not fulfill the statute” requiring notification up the chain of command, to include the White House and Congress, “or someone else at the Department of Defense withheld information from her that would have allowed her to fulfill the statute. Such disregard for clear statutory requirements is unacceptable,” the senators wrote in their letter. 

The Republicans have scheduled a press conference at noon today to elaborate on their concerns. You can watch that live on social media, here

House GOP Rep. Mike Rogers sent his own letter (PDF) demanding answers from Austin this week, too. He sent a second letter to Hicks (PDF) demanding a dozen answers from her staff; and Rogers sent a third letter (PDF) to Austin’s chief of staff, Kelly Magsamen. 

By the way: Almost a dozen lawmakers have called for Austin’s resignation, and one of those (Chris Deluzio of Pennsylvania) is a Democrat, according to Leo Shane of Military Times

The latest on Austin: He “remains hospitalized at Walter Reed National Military Medical Center and is in good condition,” Pentagon Press Secretary Air Force Maj. Gen. Pat Ryder said in a statement Wednesday afternoon. “We do not have a specific date for his release from the hospital at this time but will continue to provide daily updates until then,” he added.

U.S. Army seeks mine-clearing tech. Last month, the 20th Engineer Brigade ran a multi-day experiment at Fort Liberty, N.C., to test various options for breaching barbed wire, dragon's teeth tank barriers, and deep ditches. Some of the greatest progress so far has come from drones, which the Army uses to map out the positions of enemy mines, said Maj. Scott Rayburn, who helped lead the experiments as operations officer for the 20th Engineer Brigade. D1’s Sam Skove has more, here.

Gitmo, still open. NPR: “It was 22 years ago this week that the U.S. opened a military prison in Guantánamo Bay, Cuba, to hold suspected terrorists after the 9/11 attacks. That prison remains open today. It still holds 30 men, many of whom have never been criminally charged, and there has still been no 9/11 trial.

New: Nearly 100 advocacy organizations have sent a letter to President Biden urging him to finally close the facility.

Why has it been so hard to close the prison? More than a decade ago, D1’s Molly O’Toole (who would go on to win a Pulitzer Prize with the Los Angeles Times) explored the knotty politics surrounding the base. Read that, here.

Take a recent look inside. Here are photos and a video explainer about the detention and court operations from Carol Rosenberg, the New York Times reporter who has done as much as anyone to peer inside.