OSLO, Norway (AP) — The arrest of three suspected al-Qaida members planning bomb attacks and operating in Norway went smoothly, but a conviction might be harder to get.

Despite tightening anti-terrorism laws since 9/11, Norwegian prosecutors have yet to try a successful case, and experts say the latest arrests may run into problems faced by earlier cases.

The arrests might not end in a conviction because of the country's strict conditions for conspiracy cases, three Norwegian terror experts said Friday.

The judiciary prefers "to convict for acts rather than intention" because the latter is difficult to prove, said Tore Bjoergbo, a terrorism expert at the Norwegian Police University College.

"We tend to let 10 guilty people go free rather than put one innocent person in prison," he said.

Norway's Police Security Service arrested three residents of Norway on terrorism charges Thursday. Norwegian and U.S. officials said they were plotting to carry out an attack, organized by al-Qaida operatives in Pakistan, on an unknown target using powerful peroxide bombs similar to ones aimed for detonation in New York and Manchester, England.

A 39-year-old Norwegian of Uighur origin and a 31-year-old citizen of Uzbekistan were detained in Norway while a 37-year-old Iraqi was held in Germany, where he was vacationing, police said.

Norwegian officials allege the men conspired to commit an act of terror, a crime under Norway's 2003 anti-terrorism statute with a maximum sentence of 12 years.

Bjoergbo said much hinged on whether the suspected al-Qaida plotters had actually purchased bomb-making materials.

"If they talk on the phone or in the car about something they would like to do without making any serious preparatory acts, such as buying chemicals in a quantity that doesn't make sense for domestic purposes, then they are generally not going to be convicted," he said.

Winning a conviction in Norway, where the peculiarities of the legal system have helped thwart past trials, will likely prove much more difficult than in other European countries or in the United States.

In Norway's most notorious terrorism case, and the first to cite the 2003 terror law, three men were accused of a 2006 plot to blow up the U.S. or Israeli embassies.

Evidence included surveillance tapes of the men discussing how to attack the buildings, and prosecutors presented drawings and plans for rocket launchers.

But the court ultimately threw out the terror charges, judging that the evidence did not meet the high standards of the law.

"The threshold for being charged under (the anti-terrorism law) was designed by the lawmakers to be high," Oslo District Court judge Kim Heger said when reading the verdict for that case in 2008.

Heger said Friday that it was determined both by "a general tradition in Norwegian law" that looks skeptically on conspiracy cases and Parliament's instructions in its "written preparations for passage of the terror law."

American officials who worked on gathering evidence on the three arrested men were aware of Norway's concerns that past terror cases had fallen apart and promised to provide as much intelligence as possible to prosecutors.

Brynjar Lia, a terrorism expert at the Norwegian Defense Research Establishment, said the arrests may have been intended to break up an alleged cell rather than to secure a conviction.

"To disrupt and prevent are perhaps as much goals as to make an arrest" for Norway's Police Security Service, Lia said.

Still, Police Security Service Director Janne Kristiansen said her investigators believe they will have enough evidence to take the case successfully to trial.

"We believe we have a solid case," she said Thursday, in announcing the arrests.

Associated Press writers Matt Apuzzo and Adam Goldman in Washington contributed to this report.