This comes after the dramatic rescue of two other hostages this week, possibly putting his life at risk.
Michael Scott Moore, an author and journalist from Manhattan Beach, had been working from Berlin, Germany, and had traveled to Somalia before to conduct research. But this time, he was captured.
There are conflicting reports from Somalia about whether the pirates plan to hold him ransom or seek revenge.
Moore wrote "Sweetness and Blood," a book on surfing around the world, and it was named book of the year by the Economist in 2010. Moore's specialty is political hotspots.
The 1987 graduate of Mira Costa High School was intrigued about pirates and was in Somalia on a research mission. He was kidnapped just two days before U.S. Navy SEAL Team 6 conducted a stealth mission in the war-torn country to rescue two aid workers, 32-year-old Jessica Buchanan and 60-year-old Poul Thisted. Nine Somali kidnappers were killed during the raid.
Moore was reportedly seized by 15 men near the northern town of Galkayo, known as the "kidnap capital." He told the New York Times that the book about pirates was to expose the clash between hard fact and cliched mythology.
According to the Somalia Report, which tracks pirate activity, a pirate leader known as Mu'min said, "We don't plan to ransom them because they killed our colleagues which we are from the same tribe."
Another pirate, Hasan Abdi, told the Associated Press, "If they try again, we will all die together."
The State Department said it is in contact with Moore's family.
"We are aware of news reports that a U.S. citizen has been kidnapped in northern Somalia and we are concerned about the individual's safety and well-being," State Department spokeswoman Victoria Nuland said. "We are working with contacts in Kenya and Somalia to ascertain further information."
Meanwhile, film producer Raphael Swann was in nearby Kenya for his film, "Fishing without Nets," and provided perspective on Moore's situation.
"These guys are not good people," Swann said of the pirates. "These guys are profiteers and they are ruthless."
He is worried about what Moore is now facing.
"It's a free for all, you have the threat of the environment, which is a lack of food, a lack of water, a lack of basic human resources, combined with a civil war which has never really ended," he said.
