European officials say they are having trouble monitoring a growing roster of suspected extremists as they try to zero in on which radicals pose the greatest threat and prevent them from committing terrorist acts.

Authorities have thwarted a number of potential attacks in recent years but have failed to detect several others by assailants who were known to law enforcement but had loose if any ties to terror groups. U.S. officials face similar challenges.

Thousands of people have been flagged as potential threats, according to European authorities, including more than 3,000 in Britain and 16,000 in France.

Adding to the difficulty for authorities, a number of the recent attackers have used cars and trucks as deadly weapons, as did the man who drove into pedestrians in London on Wednesday and as others did last year in Berlin and Nice, France. The London attacker, who had a criminal record, then got out of the vehicle and fatally stabbed a police officer.

“The striking feature here is that the means of destruction were apparently so limited and basic: a man, a car and a knife,” said Timothy Wilson, a terrorism expert at the University of St. Andrews in Scotland.

In response, officials are shifting tactics. France has expanded surveillance powers for the intelligence services and police, and assigned as many as 10,000 armed soldiers to patrol potential terrorist targets.

German authorities have ramped up the vetting of refugees and deployed new software aimed at determining which of the thousands of suspected extremists they are monitoring are most likely to turn to violence.

For example, German authorities dropped surveillance of Anis Amri, who rammed a truck into a Christmas market in Berlin in December, killing 12, before his attack because he was taking drugs and drinking and had stopped talking about wanting to commit an attack, which they saw as a sign that he was becoming less dangerous. Now, such a change in behavior will be given a different weight in the risk assessment.

The U.K. has overhauled its online surveillance capabilities and substantially increased security spending.

At the same time, authorities trying to get a handle on the threat say the circle of people they are trying to track is growing.

British authorities have flagged more than 3,000 potential extremists as a concern, a British intelligence official said. “At any one time, we’re only watching a small percentage with any great resources,” the official said.

British Prime Minister Theresa May told Parliament on Thursday that U.K. security, police and intelligence agencies have successfully disrupted 13 separate terrorist plots in Britain since June 2013.

The suspect in the London attack, Khalid Masood, 52, was shot dead by police. Given limited resources, security officials could have concluded that a man of that age who hadn’t launched prior attacks wasn’t a serious threat, according to Raffaello Pantucci, director of international security studies at the Royal United Services Institute.

As of late January, France had more than 16,000 residents on a registry created in 2015 to keep track of people who may have been radicalized, the country’s then-interior minister said. Among them, 11,500 were actively monitored, he said.

This month, a Paris-born man who had been in prison for non-terrorist crimes in the past managed to grab a soldier’s assault rifle at Orly airport near Paris before being shot dead. French police had searched the man’s home after the Nov. 13, 2015, attacks in Paris that killed 130 people, but found nothing suspicious, and didn’t follow up, France’s top antiterror prosecutor said over the weekend.

Last summer, a 31-year-old man known to police as an unstable criminal barreled in a 21-ton truck through throngs of revelers on Bastille Day in Nice. It was only after the attack that investigators found that the man—who hadn’t been deemed a terror threat—had been preparing the attack for months.

In Germany, the number of Islamist extremists has risen from 100 in 2013 to 1,600 today, the country’s domestic intelligence agency said last month. Some 570 of those were considered capable of carrying out attacks, agency chief Hans-Georg Maassen said.

The inflow of more than a million refugees, mostly from the Middle East, since early 2015—many without background or even identity checks—has given authorities an additional challenge.

German officials say they are barely managing to monitor a growing pool of suspected radicals who can’t be arrested because they aren’t known to be planning an imminent attack. Deporting foreign radicals has proven difficult, in part because of the high hurdles set by German law.

Attackers who have no known links to terror groups sometimes betray common characteristics, such as “leaking violence” before an attack—getting into fights or having run-ins with the law, experts say.

But others can be relatively law-abiding, and even avoid trouble when they are under surveillance. Omar Mateen, who killed 49 people in 2016 in a shooting spree at an Orlando nightclub, had been investigated by the Federal Bureau of Investigation in 2013 and 2014, but the probes were closed after it was determined he wasn’t a threat.