KUALA LUMPUR, Malaysia » Families of the 239 people on board Malaysia Airlines Flight 370 on Sunday were marking the anniversary of the plane’s disappearance with a vow to never give up on the desperate search for wreckage and answers to the world’s biggest aviation mystery.

Voice 370, a support group for the kin of those on board, will host a "Day of Remembrance" at a mall in Kuala Lumpur. Later Sunday, the Malaysian government will release an interim investigation report, a requirement under international civil aviation regulations.

Although no wreckage has been found, officials in Australia, Malaysia and China, the three countries leading the search effort, say they are still optimistic the plane will be found in the southern Indian Ocean where they suspect it crashed after deviating from a flight to Beijing.

A year without a clue to the tragedy has frustrated the relatives. Grace Subathirai Nathan, whose mother Anne Daisy was on the plane, said Sunday’s events were important "to highlight to the public that we still don’t have any answers and that we must pursue the search."

"The lack of answers and definitive proof — such as aircraft wreckage — has made this more difficult to bear," Malaysian Prime Minster Najib Razak said in a statement. "Together with our international partners, we have followed the little evidence that exists. Malaysia remains committed to the search, and hopeful that MH370 will be found."

Foreign Minister Wang Yi of China, where most of the passengers came from, said his government will provide "all needed service to every next of kin" and help uphold their "legitimate and lawful rights and interests."

"A year has passed, the plane has not been located, but the search effort will continue," he told a news conference in Beijing. "Today must be a difficult day for the next of kin … Our hearts are with you."

In late January, Malaysia’s government formally declared Flight 370’s disappearance an accident and said all 239 people on board were presumed dead. The statement was meant to pave the way for compensation claims, but it angered many relatives who deemed it to be premature without any physical evidence of the crash.

"I want to tell the next of kin that I am also looking for the answers," Malaysian Transport Minister Liow Tiong Lai told reporters Saturday. "Our priority now is on the search ," he said, adding that if nothing is found by the end of May, "then we will have to go back to the drawing board" and come up with a new plan.

He said the safety investigation team will release its interim report later Sunday. Officials said it will detail the facts but is unlikely to touch on the cause of the tragedy.

Ships dragging sonar devices have so far scoured 44 percent of a 23,166-square-mile area off western Australia where investigators who analyzed transmissions between the aircraft and a satellite believe the plane eventually crashed after deviating from its route with its transponder and other equipment switched off.