(CNN) German police overnight thwarted a terrorist plot by a radicalized couple, a plan they suspect involved bombing a bicycle race near Frankfurt, a German terrorism researcher briefed by investigators told CNN on Thursday.

German prosecutors and police said that a man and a woman had been arrested in the Frankfurt-area town of Oberursel on suspicion of planning a Boston-style attack, but the authorities did not explicitly reveal the target.

The suspected target, according to Florian Flade, the terrorism researcher, was a race planned for Friday. The race loops around Eshborn and Frankfurt on May Day each year, attracting large crowds of spectators along the cycle route.

Prosecutor Albrecht Schreiber said police recovered a pipe bomb ready to be used, 100 rounds of 9mm ammunition, a gun, the essential parts of a G-3 assault rifle and 3 liters of hydrogen peroxide, which becomes explosive at high concentrations and has been used in multiple terror plots in the West, including the 2005 London bombings.

Earlier Thursday, Andreas Hemmes, a spokesman for the police of West Hesse, told CNN that the house and car of two individuals in Oberursel, in the forested hills west of Frankfurt, had been searched. As a result of what had been found, police had expanded their search along the L3004 road on the bike race route, Hemmes said.

"We suspect that there was a Salafist background," said Peter Beuth, the interior minister for Hesse, referring to ultra-fundamentalist interpretations of Islam. "Police investigations at this stage indicate that we have thwarted an Islamist attack."

Couple identified as being of Turkish descent

Flade, a journalist at Die Welt and terrorism researcher who first broke the story of the police raids, told CNN that a German couple of Turkish descent -- Halil and Senay D. -- were under arrest.

He said the couple had ties to radical Islamist circles in the Frankfurt area. Neither is suspected of having direct links to the leadership of a terrorist group.

Last week German police observed Halil D. moving in and out of a small forest near where he was living. They suspect he was looking for a good place to hide a bomb along the bike race route, according to Flade.

Flade said that according to German police documents, German police first became aware of the couple at the end of March when they went to a garden center near Frankfurt to purchase hydrogen peroxide. He said the store employee contacted police after becoming suspicious for several reasons.

The first was that the woman was covered in a full veil. The second was that the couple claimed they wanted to buy hydrogen peroxide to clean their fish pond in their garden, but the amount they were ordering would have been enough to clean dozens of such ponds.

Furthermore, after police thwarted a bomb plot by German extremists trained in the tribal areas of Pakistan to kill American servicemen in Germany in September 2007 with hydrogen peroxide-based bombs -- the so-called "Sauerland" plot -- German law had required such stores to report to police significant purchases of hydrogen peroxide.

Surveillance in Spain

According to Flade, after the tipoff, German investigators began trying to figure out who the couple were. All they had to go on was the surveillance footage.

The woman was fully veiled and her male companion was blurry in the tape, so they did not immediately know who they were. But in early to mid-April they were able to identify them and start surveillance to investigate the couple's radical ties.

According to Flade, German police established that the couple had recently traveled to Spain, where they met with members of Sharia4Spain, a radical pro-jihadist group linked to Al Muhajiroun in the United Kingdom. Spanish police had monitored the meeting in Spain. They also established that the couple had links to radicals who had gone to fight with AQIM, al Qaeda's North African affiliate

And they found the couple were in contact with a young radical Islamist from Frankfurt who had gone to fight in Syria at the end of last year and was recently killed.

The pipe bomb that was recovered by police appears to have similarities to devices built by Boston Marathon bombers Tamerlan and Dzhokhar Tsarnaev. Flade said the device recovered near Frankfurt included nails as shrapnel. The Boston bombers downloaded instructions from a recipe in Inspire magazine, an online Engish language magazine put out by al Qaeda in Yemen, which has also been translated into German and other languages.

In August 2013, the British security agency MI5 revealed to Parliament's intelligence and security committee that Inspire has been "read by those involved in at least seven out of the 10 attacks planned within the UK since its first issue (in 2010). We judge that it significantly enhanced the capability of individuals in four of these 10 attack plots."

Elevated threat in Germany

Like other European countries, Germany is grappling with an unprecedented terrorist threat because of the high number of its citizens who have traveled to Syria and Iraq.

In recent years there has been growing concern over radicalization in Germany's large Turkish diaspora community. Travel to Syria is particularly easy for individuals of Turkish descent because Turkey is the entry point for most foreign fighters traveling to Syria.

According to Flade, almost 700 Germans are believed to have traveled to Syria and Iraq, with up to 90% joining ISIS. One-third of these have returned to Germany and 70 to 80 have been killed in the fighting in Syria and Iraq.

There has only been one fatal terrorist attack in Germany since 9/11 -- the shooting death of two U.S. airmen outside Frankfurt Airport by a lone-wolf radicalized Islamist in March 2011.