LONDON -- After the carnage, there are questions: What can be learned from the latest London attack to prevent more of them?

Police here say they're already monitoring 500 suspects, have 3,000 other "people of interest" on their radar, and know of 20,000 other potential surveillance targets beyond that.

Richard Walton CBS News

"The volume issue has become a major challenge for the police and intelligence agencies," said Richard Walton, who used to run London's anti-terror unit.

Walton said it can take dozens of officers to watch just one suspect.

"The police and the intelligence agencies can only be devoted to the sort of top three, four, five percent of those individuals," he said. "You cannot monitor 3,000 individuals all of the time, 24 hours a day."

The so-called "new normal" keeps changing, especially as pressure on the Islamic State of Iraq and Syria (ISIS) continues to build in Syria and Iraq. The London attacks are the sting in the ISIS tail.

Sajjan Gohel CBS News

"This is a death cult, this is an entity that realizes it is losing territory in Iraq and Syria, and wants to take out as many people as it can," said Sajjan Gohel, who advises governments and police on terrorism.

And he says trying to counter ISIS propaganda doesn't work when, like the London attackers, some are just too far gone.

"There are bad people out there that have violent tendencies," Gohel said.

"They have no hesitation to kill children, to mow people down," he said. Countering violent extremism may have some place, to a degree, but it doesn't work in stopping the ideologically zealous."

What works is fast and lethal police action. But that, as we've seen, only limits the damage.

Police have started putting up concrete barriers on some of London's bridges -- protection they hope against vehicle attacks on pedestrians while they try to figure out what to do next.