(CNN) -- A potential plot against Europe was one factor contributing to the uptick this month in missile strikes by unmanned drones against terrorist targets in Pakistan, according to a U.S. official.
"We would be remiss not to try to take action to thwart what might be underway in Europe," said the official.
The official emphasized that the potential plot was not the sole factor in the U.S. decision to raise missile attacks by unmanned drones against targets in Pakistan's border area with Afghanistan to an unprecedented level.
U.S. officials say they are taking advantage of what they call "precise intelligence."
Based on information from Pakistani officials, CNN estimates there have been 20 drone attacks in the area in September alone, a higher number than in any previous month, and more than twice the monthly average.
Acknowledging the spike, one U.S. official told CNN: "Our operational tempo has been up for a while now, we have good information driving it, and given the stakes involved, we hope to keep the pressure on as long as we can."
According to the official, the mix of threats remains the same. It comes from groups like the Haqqani network, al Qaeda, the Afghan Taliban and the Pakistan Taliban. The threats they pose are "all deadly," said the official.
Earlier this month the Pakistani Taliban said it planned further attacks against Western targets. "We will launch attacks in America and Europe very soon," Qari Hussain Mehsud, a senior Pakistani Taliban leader, told Reuters by telephone from an undisclosed location.
One possible source of information on terrorists' plans is a German citizen of Afghan descent who is thought to be held in Afghanistan by U.S. forces. German intelligence officials confirm the man -- known only as Ahmad S. -- was detained in July, but they have not had access to him. They say he and several other Germans traveled from Hamburg to the Afghan/Pakistan border area in 2009, where he joined the Islamic Movement of Uzbekistan, an extremist group allied with al Qaeda.
German media reports, quoting intelligence sources, say Ahmad S. talked about possible attacks in Europe while under interrogation at the U.S. Bagram Air Force Base. But the reports did not link his information directly to the stepped-up drone campaign.
Meanwhile, a federal law enforcement official, said "the volume seems to be turned up" on the threat information coming out of Europe.
The intelligence indicates there is interest in using people with Western passports in an attack, that official said. This source says the potential operatives may be a mix of Europeans and others possibly including North Africans, Pakistanis, Turks, Uzbeks, and Tajiks.
There is concern about an "active shooter" scenario that would create as many casualties and as much chaos as possible in a short period of time. The 2008 attacks in Mumbai, India, showed how effective this kind of an attack can be in drawing attention.
According to this source, economic targets in Europe could be possible targets, including institutions such as banks and stock exchanges.
A separate law enforcement source said "the belief is" that Osama bin Laden signed off on a European atttack plan, and that source confirmed that intelligence related to a Mumbai-style attack.
The German government is increasingly concerned about the number of Germans becoming jihadists. According to a senior German counter-terrorism source, some 200 individuals have traveled to train with Jihadist groups in the Afghanistan-Pakistan border region since the 9/11 attacks.
Most of the drone attacks this year have been focused on North Waziristan, a mountainous area bordering Afghanistan where Pakistani security forces have little control. That has continued to be the pattern this month.
Pakistani officials say many recent strikes have been aimed at compounds in or around the town of Miramshah, a stronghold of the Haqqani network. Western intelligence officials have long regarded the Haqqanis as one of the most dangerous terror groups and have linked them to several attacks in Kabul.
Intelligence analysts point to other reasons for the escalated drone attacks. They include better information from sources in the border area and better surveillance technology -- including the growing use of spy balloons fitted with high-powered cameras.
In addition, the rising number of drone strikes is designed to deprive the Afghan Taliban of "strategic depth" as the Obama administration's campaign to defeat the insurgency enters a crucial phase and tighten the noose on the senior al Qaeda leadership. Pakistani officials say one strike last weekend killed Sheikh Mohammad Fateh al Masri, described as the group's senior operational commander.
CNN Homeland Security Correspondent Jeanne Meserve contributed to this report.