Police have confirmed that they are searching a number of houses in Renfrewshire in connection with the attack on Glasgow Airport.
A number of houses have been searched in Houston
The houses are in Neuk Crescent, Houston.
The airport, meanwhile, is operating a phased reopening and flights from Orlando and Ibiza have arrived.
A departure was scheduled to leave at 0900 BST to Ibiza and passengers were being processed in terminal two, airport officials said.
The reopening will be "extremely gradual", the airport officials added.
Following Saturday's incident police are now holding four people.
Strathclyde Police have linked the attack to the discovery of two car bombs in London's West End on Friday.
As a result of both incidents, the UK has moved to its highest level of terror alert - critical.
Scottish ministers met earlier for a briefing on developments.
The summit was led by First Minister Alex Salmond and attended by Scottish Cabinet Secretary Kenny MacAskill, Scotland's top law officer, Lord Advocate Elish Angiolini and the country's most senior civil servant, Scottish Executive permanent secretary Sir John Elvidge.
Scotland's equivalent of Westminster's Cobra emergency committee is the Scottish Executive Emergency Room (Seer).
Mr MacAskill is now in Glasgow for talks with police about the ongoing investigation while Mr Salmond will be meeting leaders of the Muslim community in the city later.
Mr Salmond, who has appealed for public vigilence, has said no community should be scapegoated as a result of the Glasgow incident.
Muslim leaders are holding an emergency meeting in Glasgow to discuss fallout from the attack.
Scotland's only Muslim MP Mohammed Sarwar said threats had been made towards members of the Muslim community in the aftermath of the attack in Glasgow.
He told BBC Scotland he had taken calls from people who had been threatened or targeted by abusive graffiti.
Mr Sarwar said people in the Muslim and Asian communities were "very angry".
"They're concerned about a backlash and that's why the emergency meeting has been called," he said.
Osama Saeed, from the Muslim Association of Great Britain, said: "I think, personally, it's so close to home that it's a completely different emotional reaction."
The accident and emergency department at the Royal Alexandra Hospital, Paisley, where one of the suspects is being treated for burns, is now open and fully functional.
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The suspect remains in a critical condition.
A member of the public is being treated in the Southern General Hospital in Glasgow, in connection with yesterday's attack.
He has been described as being in a stable condition.
Security has been tightened at Scotland's other airports.
The access road to Edinburgh Airport has been closed and armed police are patrolling the Forth road and rail bridges.
Lothian and Borders Police said they had also increased security at other key sites across the city, including the castle and Holyrood Palace.
At the Scottish Parliament some of the entrances have been closed with visitors going through stricter security screening.
Anyone with information should contact the Strathclyde Police incident room on 0800 0560944.