SAN DIEGO -- Three people, including a woman with ties to a Menifee home searched by the FBI in May, have been indicted on federal charges of using a weapon of mass destruction to bomb a federal courthouse and a FedEx building, authorities said Wednesday.

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

The indictment alleges that, the night of the May 4 bombing, Carlock placed three pipe bombs in a backpack, then Robinson drove with her from Menifee to the federal courthouse in San Diego.

Robinson waited in a vehicle while Carlock placed and detonated the bombs then the two drove back to Menifee, according to the indictment.

On May 16, federal agents and a Riverside County sheriff's bomb squad searched the home in Menifee on Twin Pines Street as well as a nearby field where they found what appeared to be a pipe bomb, authorities said.

No explosives were found inside the device, federal officials said at the time.

Later in the day, agents found another suspicious device in the home and that also was examined by the bomb squad, authorities said.

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.

The courthouse explosion 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.

Using surveillance video from outside the courthouse, federal investigators began piecing together evidence in the courthouse bombing and later tracked the explosive powder that was used in the bombs to a gun shop in El Cajon, said Keith Slotter, special agent in charge of the FBI's San Diego office.

"Thankfully, no one was injured," Slotter said.

Carlock, Sanders and Robinson, all of San Diego, also were indicted on charges of conspiracy to use of a weapon of mass destruction, a terrorism-related statute, conspiracy to commit malicious damage, possession of a destructive device and other charges.

If convicted of using or conspiring to use a weapon of mass destruction, they face mandatory sentences of life in prison.

Robinson, 43, was arrested Tuesday at a residence in San Diego.

Carlock, 31, was initially arrested and charged in May on an eight-count complaint charging use of false identification to obtain explosive materials, being a felon in possession of explosive materials and fraud. Carlock has a previous 1999 armed robbery conviction.

Sanders, who is at the Central California Women's Facility in Chowchilla on unrelated state charges, also faces charges of being a felon in possession of explosive materials. Sanders, 59, has a 2004 felony drug conviction.

It was not immediately known whether any of the accused had attorneys.

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

According to the indictment, Carlock, Sanders and Robinson began taking steps as early as March to buy the materials needed to build and test pipe bombs.

In April, federal investigators allege, Sanders then stole pipes and end caps typically used in commercial construction from a Home Depot store.

Robinson is accused of helping the two women steal the material.

The indictment also 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.

Staff writer John Hall contributed to this report.