Spanish prosecutors will appeal the acquittal of an alleged ringleader in the Madrid train bombing trial because he may have been cleared of one of the charges by error, officials said Tuesday.

In the sentences handed down last week, Rabei Osman, one of several alleged masterminds of the attacks and already jailed in Italy, was acquitted of the main charge of mass murder because of a lack of evidence.

The Egyptian was also acquitted of a lesser charge of belonging to a terrorist group on the grounds that an Italian court had already convicted him of the same offense. Under Spanish law, a person cannot be condemned twice for the same crime, even if courts in different countries were involved.

But a necessary condition for applying this rule is that the first conviction be definitive; in other words, the suspect must have exhausted all avenues of appeal, an official in the office of Attorney General Candido Conde-Pumpido said.

Osman was convicted in 2006 and sentenced to 10 years in jail. Last week, an appeals court upheld the conviction, but lowered the sentence to eight years. But the conviction is not definitive because Osman can still file another appeal.

Osman was arrested in Italy in June 2004 after allegedly saying in wiretapped conversations that the March 11 attacks were his idea. He repeatedly has denied it is his voice in the calls.

Because the Italian conviction is not definitive, Spanish judges may have been able to convict Osman at the Madrid trial on the terrorism charge and prosecutors at the National Court now plan to appeal, the official said.