At least five others were also charged in the sweeping investigation.
Three of Mr. Tsarnaev’s college classmates came to the attention of federal officials and eventually spent time in prison: Dias Kadyrbayev and Azamat Tazhayakov, who hindered the government’s investigation by removing evidence from Mr. Tsarnaev’s dormitory room, and Robel Phillipos, who was convicted of lying to the F.B.I.
Another man, Stephen Silva, pleaded guilty to drug and weapons charges; although he had no direct connection to the bombing, he had previously lent Mr. Tsarnaev a handgun, its serial number filed off, that was used by the Tsarnaevs to kill a Massachusetts Institute of Technology police officer.
Khairullozhon Matanov, a friend of Tamerlan Tsarnaev, Dzhokhar Tsarnaev’s older brother and accomplice in the marathon attack, admitted to deceiving investigators about his ties to the brothers and was ordered to prison.
Tamerlan Tsarnaev’s widow, Katherine Russell, was also the subject of investigation and widespread speculation in the news media, but the authorities never brought a case against her.
Dylann S. Roof, the white supremacist who killed nine black churchgoers in Charleston, S.C., in June 2015, was captured the day after his rampage. About three months later, federal agents arrested one of Mr. Roof’s friends and accused him of lying to investigators and withholding information about the planned attack.
The friend, Joseph C. Meek Jr., ultimately pleaded guilty and described how Mr. Roof had told him of his preparations for the attack, including trips to Charleston to scout out the church and his plans to carry ammunition in a fanny pack.
Mr. Meek, who was never called to testify at Mr. Roof’s trial, was sentenced to 27 months in prison. He is expected to be released in September, according to the federal Bureau of Prisons.
Mr. Roof, convicted by a federal jury in December 2016, was sentenced to death.
Despite some fears that Stephen Paddock, who opened fire in October on the Las Vegas Strip, had help from a second gunman, law enforcement officials now believe that he acted alone when he killed 58 people and wounded hundreds of others.
The investigation, though, led to the arrest of an ammunition dealer, who was accused of manufacturing armor-piercing bullets without having a proper license. The dealer, Douglas Haig, acknowledged meeting Mr. Paddock and selling him ammunition, but has said he “had no contribution to what Paddock did.”
“I had no way to see into his mind,” he said. “The product that I sold him had absolutely nothing to do with what he did. I’m a vendor. I’m a merchant whose name was released.”
Mr. Paddock’s girlfriend, Marilou Danley, who was questioned extensively after the attack, will not be charged, the authorities said in January.
San Bernardino, Calif.
A married couple’s terrorist attack in San Bernardino, Calif., in December 2015 led to a series of prosecutions, including one against a man who bought two assault rifles that were used in the rampage.
Enrique Marquez Jr. pleaded guilty in February 2017 to providing material support and resources to terrorists for his role in plotting attacks with Syed Rizwan Farook. The men never carried out their planned assaults, which targeted Riverside City College and freeway commuters, but Mr. Farook used Mr. Marquez as a “straw purchaser” to buy weapons that were later used in San Bernardino. Mr. Marquez, who faces up to 25 years in prison, has not yet been sentenced.
The federal government also secured guilty pleas — for conspiracy to commit immigration fraud — from Mr. Farook’s brother and sister-in-law. Mr. Marquez’s wife pleaded guilty to conspiracy, perjury and false statements charges that were brought when investigators discovered what they described as a sham marriage to Mr. Marquez.
Mr. Farook and his wife, Tashfeen Malik, were killed in a gun battle with the police after a pursuit.
Many investigations do not lead to charges. After a man killed 10 people at a community college in rural Oregon in 2015, for example, the authorities found that he acted alone and did not prosecute anyone. And officials in Texas have not brought any charges linked to November’s attack on a church in Sutherland Springs, in which 26 people were killed.
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