MIAMI  Jose Padilla, the Brooklyn-born convert to Islam who was once accused by the government of plotting to detonate a “dirty bomb” in the United States, was sentenced on Tuesday to 17 years and four months in prison for his role in a conspiracy to help Islamic jihadist fighters abroad.

The sentence was more lenient than the federal sentencing guidelines recommended and was a blow to the government, which had requested the maximum penalty of life imprisonment for Mr. Padilla, 37.

In explaining her decision, Judge Marcia G. Cooke of Federal District Court in Miami acknowledged the gravity of the crimes Mr. Padilla had committed. But she questioned the range and impact of the conspiracy, saying that there was no evidence linking the men to specific acts of terrorism anywhere or that their actions had resulted in death or injury to anyone.

She also noted that defendants in other well-known American terrorism cases had received life sentences for more heinous crimes, including Zacharias Moussaoui, who was convicted of conspiracy in connection with the attacks of Sept. 11, 2001, and Terry L. Nichols, who was convicted of murder in the 1995 bombing of the federal building in Oklahoma City.