BOSTON — Lawyers for Boston Marathon bombing suspect Dzhokhar Tsarnaev asked a judge on Tuesday to allow them to bolster their request to move his trial outside of Massachusetts.

In a written motion, Tsarnaev’s lawyers asked U.S. District Judge George O’Toole Jr. to give them until Aug. 7 to present an expert’s analysis of survey data and media coverage of the case to support their argument that Tsarnaev cannot get a fair trial in Massachusetts.

Tsarnaev, 20, is awaiting trial and faces the possibility of the death penalty in the deadly attack. Twin pressure cooker bombs placed near the finish line of the 2013 marathon killed three people and injured more than 260.

In a change of venue motion filed last month, Tsarnaev’s lawyers asked to move his trial from Boston to Washington, D.C. They said preliminary survey results of potential jurors in Massachusetts showed “an overwhelming presumption of guilt.”

Tsarnaev’s lawyers said they made it clear then that they still needed an expert to more fully analyze the survey data and media coverage of the case.

“The defense position is that the Court will find a pretrial decision to move the trial to a venue outside of the state of Massachusetts to be warranted once the defense is able to complete and present an expert analysis of the survey data and pretrial publicity,” they argue in the motion filed Tuesday.

Prosecutors have opposed the request, saying the poll results cited by Tsarnaev’s lawyers are “an unreliable indicator of actual jury bias.” A spokeswoman for U.S. Attorney Carmen Ortiz declined to comment on the defense request to supplement their argument to move the trial.

Prosecutors allege that Tsarnaev and his older brother, Tamerlan, carried out the bombings and killed an MIT police officer several days later.