Mr Fry appealed to Londoners to consider whether anyone who had rented accommodation recently - either for themselves or to store property - was acting suspiciously.
The device, described as a "dangerous military weapon", was found in a park in Vauxhall Cross.
It is a Russian-built Mark 22 anti-tank weapon, and the police said it was small enough to be hidden in a large holdall. They appealed for information from anyone who may have seen something suspicious.
The home of the Foreign Intelligence Service was rocked by an explosion on Wednesday at 2200 BST when a missile shattered an eighth floor window on the southern side of the building.
Click here to see location of explosion.
No group has said it carried out the attack but police suspect dissident Irish republicans.
He added: "It is a relatively portable piece of equipment... It can be carried in a large holdall, it's relatively light, its quick to operate.
"But someone saw somebody in the vicinity of Spring Gardens on Wednesday evening around 9.30pm carrying this, whether it was concealed or not, assembling it, firing it, running away in whatever direction and getting into perhaps a vehicle, perhaps a motorcycle.
Mr Fry continued: "It's designed to go through tanks and therefore it is fairly impressive against metal and it will go through about one metre of concrete."
The launcher is being examined by experts.
Police are continuing extensive searches of the area around the MI6 building.
Real IRA suspected
They have been carrying out a finger-tip search of nearby railway lines running into Waterloo station, prompting speculation the missile could have been fired from the tracks.
The missile struck just yards below sensitive satellite equipment on the MI6 building roof.
Mr Fry appealed to Londoners to consider whether anyone who had rented accommodation recently - either for themselves or to store property - was acting suspiciously.
This is the first time this type of rocket launcher has been seen and used on the mainland.
But in February this year a similar weapon was found in Dungannon and was believed to have been hidden by the Real IRA.
Mr Fry said: "This is a very easily acquired weapon if you have the mind to acquire it. It has also been made in Bulgaria and something similar has been made in Yugoslavia.
"There are stockpiles of these weapons available for those who have the intent and purpose to use them."
The Real IRA are priority suspects in the investigation into the blast, but Mr Fry has not ruled out the possibility that other terrorist groups could be responsible.
In July, the Real IRA was understood to have been responsible for a bomb in Ealing, west London, and in June Hammersmith Bridge was closed after a device was planted on one of its supports.
Security in the capital has been stepped up following Wednesday's attack.
Anyone with any information about the attack should contact the Anti-Terrorism Branch on 0800 789321.
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