It's one of the most famous unsolved mysteries in New Zealand history. In 1984, a bomb went off on the Trades Hall in Wellington killing caretaker Ernie Abbott. Former Secretary Graeme Clark remembers that day and the confusion that followed.

On the 27th of March, 1984 at the Trades Hall in Wellington, caretaker Ernie Abbott bent over to pick up a suitcase left behind earlier in the day. It contained the equivalent of three sticks of gelignite in explosive power, which were set off by an unusual switch device inside.

The 64 year-old was killed instantly.

Mr Abbott was the sole victim of the attack and despite numerous investigations, police have always struggled to identify a motive for the bombing.

They've also never laid charges for the bombing, but they believe they know who committed the crime.

In 2017, they received new information and began collecting DNA samples from suspects. Back in 1984 there were 500 persons of interest.

Detective senior sergeant Warwick McKee told RNZ's Morning Report that they were determined to resolve the investigation for Ernie Abbott and his family.

"We're pretty certain it wasn't aimed at Ernie Abbott, he was unfortunately the person that picked up the suitcase at the end of the day, being the caretaker. We believe it was targeted towards Trades Hall staff and employees."

Secretary of the Wellington Trades Council, Graeme Clark remembers that day and the confusion that followed.

Go to this episode on rnz.co.nz for more details