The FBI isn't the first intelligence agency to come under scrutiny for ignoring leads on young men who later proved to be terrorists. The U.K.'s MI5 faced similar questions after Islamist suicide bombers struck London's subway system on July 7, 2005.

The Federal Bureau of Investigation is under fire for not keeping Tamerlan Tsarnaev, who is accused bombing of Boston Marathon, under surveillance, despite a tip from Russia that he was interested in extremist Islamic groups.

There's a lot we still don't know about that episode and, as my colleague Tobin Harshaw writes, the agency needs to tell us more. Still, MI5 probably had a tougher case to answer in 2005.

While successfully tracking and preventing another bomb plot a year earlier, the U.K.'s domestic intelligence agency had monitored four phone conversations and taped two physical meetings between their top suspect, Mohammed Qayum Khan, and two unidentified young Britons who would later carry out the so-called 7/7 bombing. MI5 did almost nothing to find out who they were.

MI5 also had information that two men had traveled from the U.K. to train with Islamist militants in Pakistan and was looking for them. Yet M15 agents weren't able to link the two unidentified 7/7 bombers to the aliases that they used on their trip to Pakistan until it was too late.

Fifty-two people died in the resulting attacks, as well as the four young Britons who detonated the bombs. The bombers were three ethnic Pakistanis and one ethnic Jamaican, all born in the U.K. This attack by homegrown British terrorists came as a huge shock.

If it's any consolation to the FBI, despite these misses, a series of official inquiries found that MI5 did nothing wrong given the information and priorities it had at the time.

The reason was that the men didn't discuss any plot with Khan, the suspected bomber, in their monitored conversations. They talked about how to raise money through fraud. The car the future 7/7 bombers used was among 1,154 that crossed Khan's path in his six weeks under surveillance. His phone contacts were also numerous. Among the targets of interest MI5 had at the time across the U.K., Khan was one of the 0.13 percent that the agency had under what it considered "good" surveillance.

In other words, the unidentified fraudsters talking to Khan were a low priority. MI5 didn't have the resources or legally required evidence to put them under surveillance. To have done otherwise, the agency "would have needed to be a very different organisation, both in terms of its size and how it operates, which would have huge ramifications for our society and the way we live," said a 2009 report by the U.K. Parliament's Intelligence and Security Committee.

The 7/7 inquiries also provide an interesting blueprint of how another country responded to the sudden recognition that al-Qaeda wasn't just a foreign enemy but also a mania learned by young men bred at home.

MI5's response to its failure to detect the bombers was to hire more counterterrorism staff, with a big effort going to recruit from within Britain's Muslim communities and penetrate them. The agency rewrote its strategy for preventing homegrown terrorism, which it calls Prevent.

Two years after that revamped strategy was introduced in March 2007, it had led to: a new approach by the agency to the U.K.'s mainstream Muslim community; programs to improve the uptake of citizenship education programs in mosques and schools; a program to identify juvenile offenders vulnerable to radicalization; a major program to tackle radicalization in jails; plus 300 new officers tasked with making the policy happen.

That all sounds squishy and unwarlike. Yet MI5's focus on detecting and preventing homegrown radicalization may be more realistic than some proposals now being floated for the U.S., such as restricting immigration.

(Marc Champion is a member of Bloomberg View's editorial board. Follow him on Twitter.)