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SAN DIEGO - Three people have been indicted on federal charges of using a weapon of mass destruction in the bombings of a federal courthouse, authorities said Wednesday.

A grand jury returned the indictment Tuesday, alleging Rachelle Carlock, Ella Louise Sanders and Eric Reginald Robinson planted bombs at the downtown courthouse and a FedEx shipping company distribution center in San Diego.

The courthouse explosion on May 4 spread nails and shrapnel as far as two blocks, and authorities almost immediately began looking into whether it was related to the April 25 FedEx bombing. No one was injured in either bombing, which occurred during early morning hours.

U.S. Attorney Karen P. Hewitt would not comment on a possible motive for the bombings, saying only that the investigation was continuing. It was unclear whether others were involved or how the three people knew each other.

If convicted of using or conspiring to use a weapon of mass destruction, the three San Diego residents face mandatory sentences of life in prison. Other counts against them include conspiracy to commit malicious damage and possession of a destructive device.

The indictment alleges that the three practiced making bombs and exploding them before planting them at the courthouse and FedEx buildings. It also alleges that a little more than a week after the courthouse bombing, Carlock attempted to buy more explosive material.

The indictment alleges the two women built the pipe bombs and that Robinson helped test them.

Slotter said the courthouse explosion involved three large pipe bombs that were wrapped together and loaded with nails and explosive powder.

Authorities allege Carlock built the pipe bombs used in the courthouse attack and hid them in a black backpack. The bombs were left at the front entrance of the building.

A federal public defender entered a not guilty plea on Robinson's behalf during an arraignment Wednesday in federal court in El Centro, California.