Authorities have been taking extraordinary security measures since jury selection began in the trial of Boston Marathon bombing suspect Dzhokhar Tsarnaev (joh-HAHR' tsahr-NEYE'-ehv).

Homeland Security and Boston police are patrolling outside the federal courthouse in Boston. Armed U.S. Coast Guard boats are cruising the adjacent harbor, and officers with dogs are roaming the courtroom hallways.

The U.S. Marshals Service, charged with overseeing courthouse security, declined to comment this week on preparations.

Rick Avery of Securitas, a private security company not involved in the trial security, says he expects law enforcement to deploy bomb squads, snipers, undercover officers, enhanced surveillance operations and other measures for the duration of the trial.

Boston police say they're not aware of any credible threats.