Detectives hunting whoever sent parcel bombs to Celtic manager Neil Lennon and two others have appealed for a couple to come forward as they may have "vital" information.
The man and woman were seen getting onto a bus near to where one of the packages, addressed to lawyer Paul McBride QC, was posted.
Strathclyde Police issued the appeal as Rangers and Celtic prepare to clash for the final time this season today.
An extra 1,000 police officers are being deployed to the streets to combat a predicted surge in violence surrounding the Old Firm game.
A major police investigation is under way to trace whoever sent two packages to Lennon and one each to high-profile Celtic supporters Mr McBride and former MSP Trish Godman in recent weeks.
A Royal Mail worker intercepted the latest parcel bomb, addressed to Mr McBride, in Montgomerie Terrace, Kilwinning, Ayrshire, on April 15.
Police are trying to trace a young man and woman who were seen in the street at around 2.10pm that day getting on a Stagecoach number 20 bus which travels to Ardrossan.
The man is described as white, in his early 20s with dark-coloured hair. He was wearing a distinctive light blue and white top with horizontal stripes, and jeans.
The woman is described as white and in her early 20s, wearing a dark-coloured padded jacket.
They both got on the bus and sat together.
Police appealed for the couple or anyone who was on that bus to come forward as they may have information that is vital to the inquiry.
Detective Chief Superintendent John Mitchell, Head of Crime Division, said: "Although we have had good support from the public throughout this investigation, I am in no doubt that there are still people out there with vital information regarding this crime who, for whatever reason, have still not came forward to police.
"I am again re-appealing to anyone with any information, no matter how insignificant it may seem to them, to contact us. Please let us decide whether that information is important to our inquiries and take the appropriate action.
"Officers are continuing to follow up all lines of inquiry and I would like to reassure the public that information can be treated in confidence."
Police are also trying to trace another couple, thought to be in their late teens, and a man who were seen separately walking their dogs near the post box on Montgomerie Terrace around 2pm on Friday April 15.
Speaking ahead of today's game, Lennon said the threats would not stop him from managing the Parkhead club.
He said: "I've had this for 10 years but I don't want to say you get used to it, because you never do.
"But it is not going to deter me from doing what I want to do."
Yesterday two men were arrested following police raids targeting the homes of people allegedly involved in Old Firm internet hate campaigns.
A 23-year-old and a 27-year-old were charged with sectarian breach of the peace and are expected to appear in court on Tuesday.
The operation targeted individuals posting racial and religious hate comments about Lennon and Rangers striker El Hadji Diouf, police told the Daily Record newspaper yesterday.