Preliminary tests have found that a white powdery substance found in an envelope mailed to LDS Church headquarters is not hazardous, the FBI said.

"There is no known toxins or biological agents," FBI Special Agent Juan Becerra told the Deseret News.

The Church of Jesus Christ of Latter-day Saints' First Presidency issued a statement Friday urging respect and civility in public discourse. Gay-rights groups are also condemning the threats.

Lab tests on the substance that slipped out of an envelope opened in the annex of the Salt Lake Temple came back negative for ricin, anthrax and smallpox. Further tests were being conducted by state health officials, Becerra said.

The FBI on Friday also said it has labeled its probe into the incidents as a domestic terrorism investigation.

"Obviously it instills fear in the public eye and causes people not to feel safe," Becerra said. "It is illegal to mail something out and to threaten the use of a biological weapon or even pretend it's a biological weapon."

The LDS temples in Salt Lake City and Los Angeles received envelopes on Thursday containing a powdery substance. So did the Knights of Columbus, a Catholic fraternal organization based in New Haven, Conn. Both organizations were heavy backers of Proposition 8, the measure in California that narrowly passed Nov. 4, banning same-sex marriage.

The FBI said it is a "strong possibility" that all three letters are linked, but federal investigators reiterated they have no information linking the incidents to the proposition and its opponents.

"The evidence does not lead to that right now, and it would be irresponsible to say anything otherwise," Becerra said.

A spokesman for the Knights of Columbus told the Deseret News the letter sent to them came from California. No one has claimed responsibility for the mailings and the FBI would not say if it had identified any suspects.

In a statement, the First Presidency of the LDS Church said that since the Nov. 4 election, places of worship have been targeted with protests and vandalism.

"People of faith have been intimidated for simply exercising their democratic rights. These are not actions that are worthy of the democratic ideals of our nation," the statement said. "The end of a free and fair election should not be the beginning of a hostile response in America."

The LDS Church said it was keenly aware of the "differences of opinion on this difficult and sensitive manner," but the First Presidency expressed disappointment in what it has seen since Proposition 8 passed.