Oklahoma City (search) bombing conspirator Terry Nichols was called before a federal grand jury in Denver last week but declined to testify, his ex-wife and sister said.

Nichols is serving a life sentence without parole for his role in the 1995 bombing that killed 168 people, but a new investigation is ongoing after FBI agents discovered explosives underneath his former house in Herington, Kan. (search).

It was unclear why grand jurors were meeting or what they wanted to learn from Nichols during the appearance Thursday.

"It was very hush-hush," Nichols' ex-wife, Lana Padilla (search) of Las Vegas, told The Oklahoman in a story published Wednesday. "He told us he went in front of the grand jury, and he was kind to the people and just said this was not the right forum, this is not where he could talk."

His sister, Susie McDonnell, said: "He refused ... and gave an explanation why."

U.S. Attorney Robert McCampbell in Oklahoma City said grand jury proceedings are confidential and he couldn't comment on the newspaper's report. Jeff Dorschner, a spokesman for the U.S. attorney in Colorado, also declined to comment.

In late March, agents discovered explosives under Nichols' old house, acting on information that came indirectly from another inmate, mobster Gregory Scarpa Jr. Nichols later complained in letters to victims that Scarpa had mixed the truth about the explosives with lies in an effort to get a reduced prison sentence.

Nichols, 50, has said he would cooperate with a congressional investigation. Rep. Dana Rohrabacher, R-Calif., is considering a probe into the bombing and is seeking to meet with Nichols in prison.

McDonnell said her brother does not feel safe at the federal supermax prison in Florence, Colo.

"He feels his life is very threatened," she said. Other inmates "have told him he has been marked as a snitch and marked for murder, and so he's very fearful for his life."

She blamed FBI misconduct for the problem but would not elaborate.

"His life has been put in danger, and we don't appreciate it," she said.