A serial arsonist received a 15-year prison sentence on Friday in federal court in Boston for setting a string of fires in and around the city that caused more than $3 million in damage and injured six people, including five firefighters, authorities said.

Jose L. Baez, 41, of Dorchester, admitted to setting four fires between December 2008 and August 2010 in Cambridge and three Boston neighborhoods, US Attorney Carmen M. Ortiz’s office said in a statement. He pleaded guilty to four counts of arson in August.

At Friday’s sentencing, US District Court Judge Douglas P. Woodlock found that Baez started four additional fires in Boston between March 2006 and January 2009, though he was not charged with setting them, the statement said.

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Baez’s lawyer, Gordon W. Spencer, said in a phone interview that federal law allows judges to consider conduct for which a defendant has not been charged when passing sentence.

“It’s been contested many times, and it’s been overruled many times,” Spencer said.

He said he plans to appeal Woodlock’s denial of a defense motion to suppress evidence obtained from GPS devices that federal agents placed on two vehicles belonging to Baez without a warrant.

Prosecutors said Baez had disputes with people associated with each of the targeted buildings.

“[Baez] had to have known that the risk to human life was tremendous, particularly where he set fire to residences in the middle of the night by stacking gasoline-soaked tires and rags at the front entrances to the buildings,” prosecutors wrote in a court filing.

Spencer, in a court filing in which he requested a sentence of about seven years, said that Baez has no prior criminal record and that prosecutors are embellishing his intentions in setting the fires.

He wrote that three of the four blazes were set “in the middle of the night, when the businesses were closed, and thus one could only infer that the motivation for the fires was not to harm, but to anger. There was only one fire out of the four which appears that the intention could have been otherwise.”

According to prosecutors, the building that housed one of the three businesses, a dental office near Kenmore Square, also had apartments, and one tenant had to evacuate.

Separately, State Fire Marshal Stephen D. Coan’s office is investigating a string of arsons that have been set in vacant buildings in southeastern Massachusetts since September. Officials have said they could be the work of a single person.