It has been one of the lingering questions since the dramatic manhunt that gripped the Boston area in the days after the marathon bombings: where did the suspects get the gun that authorities say they used to kill a university police officer?

Answers may come soon with the arrest Tuesday of a man that people with knowledge of the investigation say supplied his friend Dzhokhar Tsarnaev with the weapon.

Stephen Silva made an initial appearance in federal court on charges related to heroin trafficking and possession of a handgun with an obliterated serial number.

Silva's attorney, Jonathan Shapiro, said Tuesday evening that he had received the case only a few hours earlier.

"According to news reports, law-enforcement officials say it is the same weapon that was used ... in the MIT officer Sean Collier shooting. However, this has not been charged in the indictment," he said.

Shapiro said in a statement that he was in the process of meeting with his client and reviewing evidence in the case. He declined to comment further.

The 9 mm Ruger pistol described in the indictment is the same handgun that was used to kill Massachusetts Institute of Technology police officer Sean Collier during the manhunt for the bombing suspects, according to the two people, who spoke on condition of anonymity because they were not authorized to discuss the investigation. The grand jury indictment, which was filed July 15, does not mention Collier's slaying or any connection to Tsarnaev.

The origin of the gun was among the remaining mysteries of the investigation into the April 2013 Boston Marathon attack, in which three people were killed and more than 260 were injured when twin bombs exploded near the finish line. Collier, 26, was ambushed several days later and shot multiple times in his car.

According to the indictment, Silva knowingly had possession of the gun, "which had the importer's and manufacturer's serial number removed, obliterated, and altered and had previously been shipped and transported in interstate and foreign commerce".

The indictment also alleges that Silva conspired to distribute heroin this summer in the Boston area.

Silva is a friend of Dzhokhar Tsarnaev. He said in court Tuesday that he graduated from Cambridge Rindge and Latin School in 2011, the same year as Tsarnaev. Silva was ordered to remain in custody. A bail hearing was scheduled for August 6.

Dzhokhar's brother, Tamerlan Tsarnaev, was killed in a shootout with police several days after the bombings. Dzhokhar Tsarnaev escaped but was soon found, wounded and hiding in a boat dry-docked in a backyard in suburban Watertown.

Dzhokhar Tsarnaev has pleaded not guilty and is scheduled for trial in November. He faces the possibility of the death penalty if convicted.

Four other men have been charged with crimes related to the bombing investigation.

On Monday, a federal grand jury found Azamat Tazhayakov guilty of obstruction of justice and conspiracy for trying to protect Tsarnaev by agreeing with another friend, Dias Kadyrbayev, to get rid of a backpack and disable fireworks they took from his dorm room. Kadyrbayev is to be tried next month on the same charges.

Robel Phillipos, who is charged with lying to investigators about being in the dorm room with Kadyrbayev and Tazhayakov the night the items were taken, is to have a separate trial in September. And a friend of Tamerlan Tsarnaev, Khairulluzon Matanov, is to be tried next year on charges that he lied to investigators about the extent of his friendship with Tamerlan Tsarnaev and the contact he had with both brothers in the days following the bombings.