The letters were sent to the Salt Lake City headquarters of the church, where powder spilled on a mail clerk's hand, and to a temple in Los Angeles. Both packages tested nontoxic, the FBI said Friday.
The two temples were sites of recent protests against the church's support for a California ballot initiative that superseded a court decision allowing gay marriage. The Mormon church, whose official name is the Church of Jesus Christ of Latter-day Saints, said it is stepping up security.
Church leaders released two statements Friday, one saying they were disturbed the church was being singled out for taking a position on the California amendment, the other assailing "attacks" and vandalism of church property by "opponents of Proposition 8."
"We call upon those who have honest disagreements on this issue to urge restraint upon the extreme actions of a few," church President Thomas S. Monson said in a statement.
The Utah Pride Center, a gay rights group, put out its own statement calling the powder hoaxes and acts of vandalism "deplorable."
However, the group said, "It is false to conclude that yesterday's suspicious package came from gay protesters.
Overwhelmingly, gay and allied Utahns have expressed their pain, frustration and commitment to securing rights through peaceful demonstrations and marches."
The coalition that ran the campaign to defeat Proposition 8 also issued a condemnation Friday.
"The NO on 8 campaign was about civil rights and seeking equality for all Californians. We have said time and again that the Mormon church deserves the same respect as any other religion," said Ali Bay, a spokeswoman for Equality California, the state's largest gay rights group.
The FBI is still investigating both cases, spokesman Juan T.
Becerra said, noting that it's a crime to release a substance to threaten harm and stoke public fear.
"Even if you send a hoax threat, you're still in violation of federal law," Becerra said.
Anthrax mailed as a white powder to lawmakers and media members killed five people and sickened 17 in 2001. Since then, hoaxes modeled on the anthrax mailings have popped up but usually turn out to be harmless.
Separately, the coalition of religious groups behind the successful measure held a news conference to denounce protests carried out since Election Day.
The backlash has included calls for a boycott of Utah ski resorts and California businesses whose owners donated to the cause.
"Our opponents do not like the outcome and that is to be respected. They fought hard and they feel defeated and that is understandable," said Frank Schubert, co-manager of the Yes on 8 campaign. "What they do not have the right to do, however, is to harass and intimidate people. And they do not have the right to commit acts of domestic terrorism against our supporters."
Meanwhile, five civil rights groups asked California's highest court Friday to annul the ban on the grounds that Proposition 8 threatens the legal standing of all minority groups, not just gays.
The NAACP Legal Defense and Educational Fund, the Mexican-American Legal Defense Fund, the Asian Pacific American Legal Center and two other groups petitioned the state Supreme Court to prevent the change from taking effect.
The petition is the fourth seeking to have the measure invalidated. But it's the first to argue that the court should step in because the gay marriage ban, which overturned the Supreme Court ruling that legalized gay unions, sets a precedent that could be used to undermine the rights of racial minorities.
Eva Paterson, president of the San Francisco-based Equal Justice Society, said the election raises the specter of voters deciding to bar illegal immigrants from public schools, disenfranchising black voters or otherwise using the ballot box to promote segregation.
"The court ruled that to discriminate in the area of same-sex marriage was unconstitutional and violated our guaranteed equality," Paterson said. "Why should a slim majority of Californians be able to put discrimination back into the California Constitution?"