Washington (CNN)The colossal challenge confronting officials trying to prevent and track an attack like the one in Paris boils down to this: There are too many people to watch.
A problem for terror investigators: Too many to watch
An estimated 5,000 French citizens are now under some form of direct observation or surveillance by French security services because of their involvement in various jihadist movements. But those 5,000 are just the cases that French authorities know about.
Many officials suspect there are many others who are radicalized in some fashion but have not made it on to the radar in France.
Some of these individuals under surveillance are fighters who have returned home after doing battle with groups like ISIS in Syria and Iraq. Others are known to authorities from their activities within France. But the enormity of the numbers poses a problem for governments working to stop the next attack.
And the issue grows exponentially across the European continent when you take into account the estimated 3,000 fighters in Iraq and Syria that officials say come from various European countries.
"European security services are just stretched almost beyond breaking point right now," says CNN Terrorism Analyst Paul Cruickshank. "They just don't have the resources to monitor all these people."
A former senior French counterterrorism official tells CNN that surveillance on an individual in France requires three to ten agents for round the clock surveillance.
"There are too many of them, too few of us," the official said.
"Dozens of people are being followed, dozens of people have been detained. Dozens of people have been imprisoned." French Prime Minister Manuel Valls said Thursday. "But what I see is the demonstration of the difficulty confronting our services: the number of people who represent a danger."
Cherif Kouachi, one of the two suspected gunmen in the Paris attack, was at one point under surveillance by the French government for his role in a jihadist recruiting ring, but that surveillance had stopped prior to the attack in Paris.
Some 200 French nationals have come back home after fighting in Syria, some of them subsequently incarcerated for one reason or another, but many others remain free because they have not yet crossed a legal line.
But governments cannot keep a 24/7 watch on every returning fighter, and that can pose problems.
At one point, German authorities alerted the French government about the presence of Mehdi Nemmouche, a French-Algerian who at one point fought for ISIS in Syria, who was in Frankfurt. He also traveled to France at some point as well, before he allegedly shot and killed four people in May at a Jewish museum in Brussels.
Law enforcement officials in the United Kingdom warned during this past holiday season that they felt they were short of the necessary manpower needed to address the overall terror threat posed by people they were watching in that country.
"In a large country and an open and democratic society, it's difficult to keep track of every single one of these guys that comes back," says CNN National Security Analyst Fran Townsend.
There are about a dozen Americans involved in the fighting in Syria so U.S. officials do not have the same challenge facing their European counterparts on that front. But like the attack at the 2013 Boston Marathon, the attack in Paris may have been perpetrated by brothers - an altogether different type of challenge for authorities.
"Once you are inside a family, these guys don't need to call each other, they don't need to communicate over the internet," Townsend said. "That's what becomes virtually impossible to disrupt, unless there are more involved."