TAMPA — The remains of a mother of five who vanished nearly 18 months ago have been found, but the mystery of just how she died is yet unanswered.The discovery Saturday by Tampa police brings some relief to family members who have searched for Misti Whitfield, 35, her stepfather said. But questions about how and why she disappeared linger.Whitfield was last seen alive about 10 p.m. May 2, 2013, walking near 5525 W Hillsborough Ave., north of Tampa International Airport. Police officials declined to say Thursday where and how her remains were found.Authorities have been unable to determine how she died, but said the investigation is ongoing. Police are releasing few details.Whitfield's stepfather, Mel Ardelean, called the news "bittersweet." Searching for their daughter every day for nearly 18 months was agonizing, he said."That's the sweet part of it, that finally that agonizing process is ended," he said. "The bitter part is now we're in the phase of, 'What happened to her, if something did happen to her, who did it and why.' "Ardelean said Whitfield's remains were found close to where she disappeared, but police have not been specific."We've learned to be patient," he said. He declined to say much about Whiftfield's children, but said they are okay.Ardelean and his wife, Sharon, of Plant City became worried after Whitfield did not contact them on Mother's Day that year, police said. They reported her missing on May 14, 2013."The family suspected foul play from the minute she didn't call on Mother's Day," Ardelean said.When she disappeared, police said Whitfield had previously struggled with alcohol and drug addictions.Her parents organized a group of friends and volunteers to search for their daughter. They donned rubber bracelets bearing the words "Mission for Misti." They hired a private investigator and commandeered their daughter's Facebook page.When Whitfield was last seen alive, she was arguing with a man, her family later said. Her bicycle was left outside Nautical Marine, a boat-repair business. It was returned to her apartment 10 miles away by that same man two days later, the family said.A memorial service has been scheduled for 5 p.m. Sunday in McCall Park in Plant City. A reception will follow at Cuzzins Bar. Times staff writer Keeley Sheehan contributed to this report.