Updated at 7.20pm with Prime Minister's tribute

Karin Grech was killed through a letter bomb on December 28, 1977... and 40 years later her case remains one of most elusive in Malta's criminal history.

The 15-year-old girl was killed when a parcel addressed to her father, a doctor embroiled in the midst of an acrimonious strike, blew up in her hands, sparking a politically-dark and violent period.

In the presence of her 10-year-old brother, Karin opened the large brown package, which she thought to be a present since it was covered in Christmas wrapping paper. In the package was a small, pen-box shaped parcel which blew up once opened. Her body succumbed to the burns half an hour later at St Luke's Hospital.

At her funeral Mass, Archbishop Mikiel Gonzi had called the murder "the first terrorist act in the country".

The brown envelope which contained the explosive device.

The girl had taken the hit for her father Edwin Grech, then head of the department of Obstetrics and Gynecology at St Luke's Hospital.

The case dates back to August 1977 when Prof. Grech was working as an obstetrics and gynaecology consultant in the UK when doctors in Malta were taking industrial action following disagreement between the government and the Medical Association of Malta.

The main bone of contention was a two-year housemanship imposed on new doctors before getting their warrant. Medical strikes followed and the Labour government retaliated by locking strikers out of hospital. The medical course was also impacted and students were forced to study abroad.

Read: The bomber who killed Karin Grech was a 'loner'

The government asked Prof. Grech to return to Malta to head the Obstetrics and Gynaecology Department. He agreed to do so for the duration of the industrial dispute and as a result he was labelled a strike-breaker.

On the same day that the Grech family received the bomb, another bomb was sent to the doctor and Labour MP at the time Paul Chetcuti Caruana, but it failed to detonate.

Despite fingers being pointed at many suspects, the perpetrators remain unknown, and the magisterial inquiry is still open.

The explosive device was aimed at Prof. Edwin Grech, pictured here in 2011.

Speaking in 2011, Prof. Grech said he had information that the explosive device had been planned by fourth- and fifth-year medical students who hired a criminal to make the bomb that was delivered by a carpenter with missing fingers. His claims, however, never yielded any suspects.

Just seven years ago the Civil Court awarded compensation of €419,000 to the Grech family.

A judicial protest was filed last month by the Grech family formally holding the Attorney General, Commissioner of Police and the Director General of Courts responsible for the disappearance of a crucial piece of evidence: pieces of the envelope which contained the explosive, which had apparently gone missing from the courts. However, the court was later informed that the evidence had since been found and investigations could continue.

Karin Grech's murder remains one of the most notorious murders with political undertones in Malta's recent history, alongside the killing of Raymond Caruana in 1986 and Daphne Caruana Galizia last October.

This evening, Prime Minister Joseph Muscat paid tribute to Ms Grech at her monument in San Ġwann.

Describing her and Mr Caruana as siblings, he said they were victims.

Dr Muscsat also referred to this year’s murder of Ms Caruana Galizia and said the difference between the three was that people had been arraigned in court for the latter’s murder and justice should now be allowed to take its course.

The state, he said, wanted to find the hidden hand behind these homicides.

Flowers were also laid at Ms Grech’s monument by Alternattiva Demokratika chairman Carmel Cacopardo.