The potential witnesses were seen near the post box in Montgomerie Terrace Police investigating parcel bombs sent to Celtic manager Neil Lennon and two prominent supporters of the club want to trace three potential witnesses.
A couple, thought to be in their late teens, and another man, were seen separately walking near a post box in Kilwinning, North Ayrshire, on Friday.
The post box in Montgomerie Terrace is a “focal part” of the investigation
The devices were addressed to Mr Lennon, Paul McBride QC and former Labour MSP Trish Godman.
Police said the potential witnesses were possibly near the post box at about 1400 BST on 15 April.
The man is described as being in his 30s, wearing glasses and a baseball cap and was walking with a black and brown Staffordshire terrier type dog.
The young couple are described as a man who was wearing a black hooded top and a woman who was small and slim.
They were walking with a dark-coloured Staffordshire Terrier type puppy, thought to be about eight weeks old.
Devices were sent to Neil Lennon (L), Trish Godman and Paul McBride QC
Who are the parcel bomb targets? Reaction to Lennon bomb threats
Det Ch Supt John Mitchell, of Strathclyde Police, said: “As part of our ongoing investigation we have been conducting house-to-house inquiries in Montgomerie Terrace in Kilwinning and as a result, we have identified three people that we would like to speak to as potential witnesses.
“We would like these potential witnesses to come forward as they could have vital information which might help our investigation.
“Anything they may have seen or heard on the street at that time could provide us with clues which could lead us to the person or people responsible for these crimes.”
Det Ch Supt Mitchell added: “The person or persons who are responsible for this have not just caused massive distress and worry to the intended targets, they have also placed the people who handle the mail in harms way.
“They must be caught and they must be brought to justice.”
The first device was intercepted by the Royal Mail in Kirkintilloch, East Dunbartonshire, on 26 March and was addressed to Mr Lennon at Celtic’s training ground in nearby Lennoxtown.
Two days later, a device was delivered to Labour politician Ms Godman’s constituency office in Bridge of Weir, Renfrewshire. Her staff were suspicious and contacted Strathclyde Police.
The third package was addressed to Mr McBride at the Faculty of Advocates in Edinburgh.
David Cameron condemned the threats to Neil Lennon and two Celtic fans It is believed to have been posted in Ayrshire, before being found in a letter box by a postal worker on Friday and taken to a Royal Mail sorting office in Kilwinning, where police were contacted.
Detectives are also investigating another package addressed to Neil Lennon which was found at a sorting office in Saltcoats, North Ayrshire, on 4 March but this has not been confirmed as an explosive device.
Ch Supt Ruaraidh Nicolson said: “I would like to make it absolutely clear that the people who have received these packages appear to have been targeted for comments they have made in recent weeks.
“We do not believe that this is a general threat to all Celtic supporters or other high profile supporters of the club.
“We are giving appropriate advice to people who we believe should be getting it.”
Earlier, David Cameron described the parcel bomb threats as “an appalling act”.
The prime minister, who is visiting Scotland ahead of next month’s Holyrood election, said police would be given every help to catch those involved.
“Any assistance the Strathclyde Police need the Strathclyde Police shall get because this is an absolutely appalling act,” he said.
“The most important thing is that the police pursue it with every piece of vigour they have and get to grips and find the person who is responsible for it and [ensure] they are severely punished.”
He added: “It is a reminder of the appalling sectarianism that exists in some people’s minds, even as we actually deal with it quite effectively in Northern Ireland, it’s still a problem and it must be sought out and crushed.”
Meanwhile, it has been reported that a live bullet was sent to the official residence of Scotland’s most senior Roman catholic, Cardinal Keith O’Brien, ahead of the Pope’s visit to Scotland last year.
The incident, which has not being linked to the parcel bomb or bullet threats to Celtic-related figures, was not reported at the time.
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