NAIROBI — For years, it appeared that the world's efforts to combat Somali pirates had paid off. The number of ship attacks plummeted. NATO's counter-piracy operation was so successful that officials decided in December that it was no longer necessary.

Then, on Monday evening, a tanker called the Aris 13, carrying fuel from Djibouti to the Somali capital of Mogadishu, sent out a distress signal. It was being taken over by armed men. For the first time since 2012, a commercial ship had been attacked by Somali pirates.

Now, experts and officials are trying to figure out what the newest abduction says about efforts to secure one of the world's most dangerous maritime routes. So far, one thing appears clear: Both Western naval fleets and commercial shipping companies underestimated the threat still posed by Somali pirates.

The Aris 13 drifted toward the Somali coast near the far eastern tip of Africa, possibly in an attempt to save time, according to John Steed, an expert in maritime crime at Oceans Beyond Piracy, a nonprofit group based in Colorado. That decision put the crew in great danger.

Making it even more vulnerable, the Aris 13 had no armed guards, and there was no barrier blocking would-be assailants. As the risks facing ships appeared to decline in recent years, many vessels appeared to loosen their security plans.

“One of the main problems with deterring piracy is only having 35-40 percent of ships in the High Risk Area with armed guards on board commercial vessels,” said Chris Suckling, an analyst at IHS Markit, which tracks the maritime industry.

NATO's withdrawal from the waterway didn't help, either. Although the European Union and often the United States conduct their own counter-piracy operations in the Gulf of Aden, NATO's Operation Ocean Shield was a key deterrent from 2009 until December.

“The very presence of this international naval force deterred pirates from pursuing their activities and contributed to the suppression of piracy in the region” said a NATO statement last year.

But despite the success of the naval operation, the part of northern Somalia known for piracy remained poorly governed and plagued by fighting between rival clans.

According to Suckling, pirates did not disappear. Rather, they shifted their focus to other illicit activities, such as arms trafficking to Yemen. One theory, he said, is that a clan opposed to the president of Somalia's Puntland region encouraged the attack on the Aris 13 to promote the idea that the area is poorly governed.

“There’s a perception that the capability isn’t there, but it’s just that motives have changed to being political rather than primarily economic,” Suckling said.

Somali fishermen have said the attack on the Aris 13 was an attempt to end the scourge of illegal fishing, mostly by Asian ships, in their local waters.

“So this is not a pirate attack, it's angry fishermen trying to protect their area so that they can fish,” said Harare Ahmed Mohamed Matan, a fisherman in Puntland, in a phone interview. “That livelihood has been disrupted by constant threats from these ships fishing illegally.”

The Aris 13 wasn't a fishing vessel, but Somali fishermen have suggested that abducting its crew could be an effective way to bring attention to their complaints and deter foreign fishing boats.

The E.U. has said that the pirates are interested in negotiating a ransom. The Sri Lankan government said that eight of its citizens were on the ship.

On Tuesday night, Steed said, the ship was moved to the coastal town of Xabo.

“Usually pirates reinforce with guards from the shore and use the village to resupply,” Steed said.