Regelin, 24, who grew up in Anderson, Calif., was assigned to a San Diego unit and was working with a U.S. Marine Corps company when he was fatally wounded Monday.
He was standing in for a wounded bomb technician on foot patrol when an explosion occurred. When he went to investigate, a second bomb was detonated nearby.
A Navy spokesman told The San Diego Union-Tribune an improvised bomb killed Regelin, but could provide no further details.
Regelin was described by his brother Ryan as intensely committed to his job, noting he had volunteered for the tour of duty in Afghanistan. Days before his death, he had re-enlisted for another six years.
"He was a thrill junkie," Ryan Regelin told the Union-Tribune. "He said he hates sitting at home. He loved what he did."