The bomb attacks in London on 7 July 2005 may have been stopped had security had more resources, which means things like staff, money and information.
A report written by MPs said that two of the men who carried out the attacks were known to security services.
But the MPs behind the report said it was "understandable" why the two men were not investigated more.
It added had there been more resources available to security services, the attacks might have been stopped.
The attacks on London on three tube trains and a bus killed 52 people.
The report said the security services had come across the lead bomber Mohammad Sidique Khan, 30, and fellow bomber Shezhad Tanweer, 22, while investigating other cases.
Investigations into some of those other cases stopped different planned attacks on the UK.
The report reads: "If more resources had been in place sooner the chances of preventing the July attacks could have increased."
There's also a recommendation that the way that potential British terrorist threats are treated is changed in future.