US Attorney Carmen M. Ortiz said this week that she has appointed William Weinreb, a career prosecutor who led the team that secured a death sentence for Boston Marathon bomber Dzhokhar Tsarnaev, as her first assistant United States attorney.

As first assistant, Weinreb will be Ortiz’s go-to person on legal affairs including complex civil matters and questions of law in criminal indictments. His appointment comes as Ortiz’s office is involved in several high-profile matters, including a civil rights investigation into allegations of racism at Boston Latin School, and a criminal investigation into alleged union strong-arming.

“Bill has served as a trusted advisor to me, sharing his expertise and legal acumen cultivated by years as a litigator, on numerous sensitive legal and law enforcement matters handled by the office,’’ Ortiz said in a statement. “Bill’s longstanding commitment to public service, combined with his experience, will serve the district of Massachusetts well, and I look forward to working closely with him in his new role.’’

Weinreb previously served as counsel to Ortiz for two years, and was deputy chief of the office’s National Security Unit and coordinator of the Anti-Terrorism Advisory Council. He replaces John McNeil, who was assigned to the National Security Unit in the Boston office.

In a statement, Weinreb said “I am proud to help lead an office of dedicated professionals who make the Commonwealth a better place to live by combatting crime, reaching out to vulnerable communities, and protecting the rights of residents.’’

In addition to the conviction of Tsarnaev, which led to the first death sentence in a terrorism case after 9/11, Weinreb has also been involved in other high-profile cases. He received the Attorney General’s Distinguished Service Award for his prosecution of Aftab Ali, a Boston resident who helped fund the attempted car bombing of Times Square in 2011.

He also obtained convictions in Massachusetts’ first case of foreign economic espionage and in the district’s first case involving the use of a laser to interfere with the operation of an aircraft, according to the US attorney’s office. He also prosecuted the first case involving the threatened use of a biological weapon — ricin — and has been involved in other cases involving violent crime, public corruption, and insurance fraud.

In addition to prosecuting criminal cases, Weinreb also helped launch the office’s new Civil Rights Unit, which is investigating allegations of discrimination at Boston Latin School, and whether the town of Dudley violated the religious freedom of area Muslims by blocking the construction of an Islamic cemetery.

Milton J. Valencia can be reached at milton.valencia@globe.com. Follow him on Twitter @miltonvalencia.