Missing Malaysia Airlines flight 370 was carrying more than two tonnes of a cargo which has never been fully identified, relatives of those on board claim.
Voice 370, a lobby group made up of MH370 families, called on the Malaysian government to release the plane's full cargo manifest in a letter sent to new Prime Minister Dr Mahathir Mohamad and transport minister Anthony Loke Siew Fook.
It comes as Malaysian authorities revealed the physical search would end next week.
Danica Weeks, the wife of New Zealand MH370 passenger Paul Weeks and mother of their two young sons, confirmed the group was troubled by 2.2 tonnes of cargo described as "radio accessories and chargers" that was mysteriously omitted from the manifest.
"It's just incredible that we have a situation where an airline has taken people's money to fly them from one destination to another and we still don't know exactly was in that plane's cargo," Ms Weeks told news.com.au.
"I mean it's been four years and we have still not seen the full manifest. We still don't know what 'radio accessories' means. We have asked repeatedly for a full list of everything that was on that plane and the airline has been unable to produce that."
In the letter, Voice 370 calls for "a comprehensive review of all matters related to the disappearance of MH370, especially the release of all relevant documents such as the full cargo manifest; an investigation into any possible falsification and or elimination of records related to MH370 (and) its maintenance".
The group also calls for a "further investigation and inquiry into any act or omission across the entire spectrum of operations that might have impaired tracking, search, rescue and recovery".
The mystery cargo was the focus of early conspiracy theories relating to MH370's disappearance thanks to a decision by Malaysian authorities not to immediately disclose that the plane had a been carrying a large quantity of potentially flammable lithium batteries.
Malaysia Airlines (MAS) former CEO Ahmad Jauhari Yahya revealed there were 200kg of lithium batteries on board at a press conference on March 20, 2014.
Malaysia also delayed the release of what they claimed was the full cargo manifest until two months after the plane vanished en route from Kuala Lumpur to Beijing on March 8, 2014 with 239 people on board.
Incredibly, once the document was made public, it became apparent that there was a glaring inconsistency between the stated gross weight of the shipment and weight of the batteries according to MAS.
Page five of the manifest lists 200 units of lithium ion batteries as a "consolidated consignment" weighing a total of 2453kg. The batteries were sent from NNR Global Logistics in Penang to JHJ International Transportation in Beijing.
The airline later released a statement admitting the batteries made up only a small fraction of the total shipment and identifying the rest of shipment — all 2.2 tonnes of it — as "radio accessories and chargers".
"About two tonnes, equivalent to 2453kg of cargo was declared as consolidated under one master airway bill (AWB)," MAS said on May 2, 2014.
"This master AWB actually comprised five house AWB. Of these five AWB, two contained lithium ion batteries amounting to a total tonnage volume of 221kg. The balance three house AWB, amounting to 2232kg, were declared as radio accessories and chargers."
However, there is no mention of radio accessories and chargers anywhere on the manifest and that discrepancy, whether down to carelessness or something more sinister, was a matter of great concern to the passengers' next of kin, Ms Weeks said.
"That fact that we still haven't found the plane and we still don't know what happened means that it could happen again at any time," she said.
"Eight million people are flying every day and the risk is still there. The risk will always be there until we know what happened.
"I hate flying, [my son] Jack says he hates planes and of course I won't fly without them now in case it happens again. I mean it still feels like yesterday. I still haven't had a memorial or anything for [husband] Paul and I can't until I have some idea what happened and where the plane is.
"It just doesn't stop, the pain. It doesn't get better as time goes on. It gets worse."
Ms Weeks said she was hopeful that the new regime in Malaysia would be more transparent than the previous Razak government and said that she was heartened by newly minted transport minister Anthony Loke Siew Fook's recent pledge to find the truth about MH370.
"I hope the ministry will be able to bring closure to the families of those on board as well as resolve one of the biggest mysteries in the aviation industry during my tenure," Mr Loke told The Star online.
Australian Transport Safety Bureau (ATSB) officials Greg Hood and Peter Foley, who co-ordinated the three-year, Australia-led search for the plane, have rejected claims made by former pilots Simon Hardy and Larry Vance that MH370 Captain Zaharie Ahmad Shah downed the plane in an act of murder-suicide.
"There's no earthly reason why someone in control of an aircraft would exhaust its fuel and then attempt to glide it when they have the option of ditching," Mr Foley told a Senate estimates hearing on Tuesday.
"The aircraft was probably descending in an uncontrolled manner."