BERLIN – German authorities were searching today for about 10 suspected supporters of an Islamic group linked to Al Qaeda believed to have assisted three militants arrested for plotting imminent attacks against Americans in Germany, an official said.

Authorities believed “some 10” further suspects provided support to the two German converts and a Turkish citizen who were arrested Tuesday, August Hanning, a top security official, told the ARD broadcaster.

“This is the network that we are aware of at the moment,” Mr. Hanning said, adding that authorities believe the splintered cell — which includes more converted Germans, Turks and other citizens — no longer poses a direct security threat.

The three arrested suspects had military-style detonators and enough material to make bombs more powerful than those that killed 191 people in Madrid in 2004 and 52 commuters in London two years ago, prosecutors said yesterday.

“We were able to succeed in recognizing and preventing the most serious and massive bombings,” German Federal Prosecutor Monika Harms said at a news conference Wednesday. She declined to name specific targets.

In Washington, a senior American State Department official said German investigators had determined the Frankfurt International Airport and the nearby American Ramstein Air Base were the primary targets of the plot, but that those arrested may have also been considering strikes on other sites, particularly facilities associated with America.

The official spoke on condition of anonymity to describe discussions between American and German intelligence agencies.

Germany’s announcement was the second in two days that a major attack had been foiled in Europe, after Danish authorities arrested eight alleged Islamic militants with links to senior Al Qaeda terrorists.

The German raids were launched after an intense, six-month investigation by 300 officers, who followed the suspects so closely that, at one point, police stealthily substituted a harmless alternate for the raw bomb material the suspects had collected, according to prosecutors.

German and American officials have been increasingly on edge after Islamist attacks on German troops in Afghanistan, fearing an attack at home, and security measures had been increased.

Germany’s elite GSG-9 anti-terrorist unit arrested two of the suspects Tuesday at a vacation home in Oberschledorn, a town of some 900 people in central Germany. A third suspect fled through a bathroom window, but was caught about 300 yards away, authorities said.

The suspects were brought before a judge in closed-door sessions Wednesday at the Federal Court of Justice in Karlsruhe, and were ordered held pending trial.

Prosecutors said the three — whom they identified only as Fritz Martin G., 28, Adem Y., 28, and Daniel Martin S., 21 — had first come to the attention of law enforcement when one or more of them carried out surveillance of U.S. military facilities in Hanau, near Frankfurt, in late 2006.

Over the course of the next six months, authorities observed them gathering a dozen containers of 35 percent hydrogen peroxide solution, which officials said can easily be combined with other material to make explosives.

Police decided to move in when the suspects began moving some of the containers and acquiring other equipment used to make bombs.

Prosecutors said the three had undergone training at camps in Pakistan run by the Islamic Jihad Union, and had formed a German cell of the al-Qaida-influenced group.

They described the Islamic Jihad Union as a Sunni Muslim group based in Central Asia that was an offshoot of the Islamic Movement of Uzbekistan, an extremist group with origins in that country.