“This is probably the DNA of the perpetrator of the crimes,” Zurich's public prosecutor said when finally releasing the information in a statement on Thursday.
The DNA match has been kept under wraps until now to avoid jeopardizing the two investigations, said the statement.
However it is now being made public in the hope that someone may come forward with new information.
DNA belonging to an unknown male was found at the scene of the killing of a 56-year-old psychiatrist in her practice in the Seefeld area of Zurich on December 15th 2010.
In 2011 around 300 men were DNA tested by Zurich cantonal police, the first such event of its kind in Switzerland, but no match was found.
Exactly five years after the Seefeld homicide, a 64-year-old woman and her 74-year-old husband were killed in their home in Laupen, in the canton of Bern. They were found on December 18th, but the crime was shown to have taken place three days earlier, on the 15th.
Police analysis of DNA trace material at the scene uncovered the match with the Seefeld murderer.
“The identical DNA is, for the time being, the only concrete link between the two killings,” said the public prosecutor. The statement did not address the identical date.
Investigators are hoping that releasing news of the link will prompt members of the public to come forward with potentially new information.
Both cases posted rewards for information leading to the apprehension of the killer – 20,000 francs in the Laupen case and 10,000 for the Seefeld murder – therefore both could be paid out if the unknown killer is caught.
Anyone with information is asked to call Zurich police on 044 247 22 11 or Bern police on 031 634 41 11.