What happened next to the MV Maersk Tigris, however, was quite out of the ordinary.

“The master was contacted and directed to proceed further into Iranian territorial waters,” said Warren. “He declined and one of the IRGCN craft fired shots across the bridge of the Maersk Tigris. The master complied with the Iranian demand and proceeded into Iranian waters in the vicinity of Larak Island.”

William Watson, a maritime consultant based in Washington, D.C., called the situation “very strange and peculiar.”

Iran, which claims the entire strait as its territorial waters, might legally board a vessel if it deviated substantially toward the Iranian coast, Watson said. But ships moving normally through the strait have the right of innocent passage, a right routinely and firmly asserted by U.S. warships, among thousands of other vessels.

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Via its Fars News Agency, the Iranian government said , “ The ship is a trade vessel and has been seized by the Iranian navy at the request of Iran's Ports and Maritime Organization...The ship was seized after a relevant court order was issued for its confiscation." The article said the IPMO had monetary differences with the ship owner.

Watson found this mystifying. If someone has a financial claim against a vessel’s owners, the claimant can “arrest” the vessel, or hold it until the dispute is resolved. But he added that in his decades of watching the world’s maritime trade, he’d never heard of such a thing done on the high seas. Arrests happen in port or at anchor, he said.

Soon after the container ship encountered the IRGCN boats, it sent a distress signal. The U.S. Navy responded by dispatching a guided missile destroyer, the USS Farragut, to have a look. As well, it sent a maritime patrol aircraft (the Navy has two kinds, the propellor-driven P-3 Orion and the jet-powered P-8 Poseidon).

It’s unclear what the Navy might do from here. The U.S. can act forcefully to protect ships under U.S. flag, and generally must lay off when a vessel is sailing under some other country’s banner. The Maersk Tigris is a bit in the middle; it flies the flag of the Marshall Islands, which in the wake of World War II placed itself under the military protection of the United States.

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“ NAVCENT [U.S. Naval Forces, Central Command] is communicating with representatives of the shipping company and we continue to monitor the situation,” Warren said. “According to information received from the vessel's operators, there are no Americans aboard.”

The incident comes just days after the U.S. Navy dispatched an aircraft carrier and escort to ward off Iranian ships headed for the civil-war-wracked country of Yemen, and amid tense and ongoing negotiations surrounding the framework nuclear deal between Iran and other nations. It is also part of a long history of naval confrontations between the U.S. and Iranian forces; most dramatically, the daylong naval battle in 1988 in which the U.S. retaliated for the mining of the USS Samuel B. Roberts by sinking two Iranian warships and damaging other assets.

Update (4/29): The day after the seizure, Maersk officials told Reuters they still did not know why their ship had been taken, and that they were working with Danish diplomats to learn more. The world's largest shipping company, Maersk is based in Copenhagen.

Update 2 (4/29): Via the government's IRNA news agency, Iran added a bit to its explanation for the seizure, saying that “ the decree was issued upon a complaint lodged by a private company named 'Pars-Talaeeyeh Oil Products Company ' against MAERSK Shipping Line. The case passed its legal proceedings and finally MAERSK was sentenced to pay financial damages.... The [ Navigation and Ports Organization] underlined that the issue is merely a legal case and has nothing to do with political issues. ”

Update 3 (4/29): The website MarineTraffic produced this video showing the course of the MV Maersk Tigris before, during, and after its interception.

Update 4 (4/29): Speaking in New York City, Iranian Foreign Minister Mohammad Javad Zarif reiterated that the ship was seized as part of a financial dispute between private companies, and said that the incident has nothing to do with international relations, McClatchy reports .

Update 5 (4/30): New York Times : “ Iran’s seizure of a container ship this week stems from a decade-long dispute over 10 shipping containers, officials at Maersk, the Danish shipping giant, said on Thursday. ”

Update 6 (4/30): IHS Maritime 360 : “ Maersk representative Michael Christian Storgaard said it was 'illegal from a UN point of view to seize a commercial vessel ’ while in international waters or while making innocent passage through a country's territorial waters... ' When someone seizes something, you are told the reason. You are presented with some court ruling or an arrest order or an official document, ’ he said. ”

Update 7 (4/30): Col. Warren, the DoD spokesman, told reporters this morning that the Tigris remains at anchor some 15 to 20 miles off the Iranian port. Meanwhile, the U.S. government is in discussion with the Marshall Islands on the way forward, and t he Pentagon is continuing to monitor developments. The U.S. destroyers are still in the area; the Farragut is sailing some 60 nautical miles away from the Tigris, and U.S. aircraft continue to monitor from the skies.

Update 8 (4/30): The Pentagon said today that U.S. Navy warships would begin to accompany U.S.-flagged vessels moving through the Strait of Hormuz.

Kedar Pavgi and Ben Watson contributed to this report.