Abortion clinics around the nation are among the latest targets of anthrax scares, with more than 100 of them reportedly getting white powder-filled envelopes in the mail this week.

Administrators say that the envelopes turned up in about 110 clinics and Planned Parenthood facilities in the country on Monday. Of the pair of letters tested, neither was found to contain anthrax, according to authorities.

"The envelopes were very professional and had law enforcement addresses on them," said Planned Parenthood President Gloria Feldt. "They had the words: 'time sensitive security information' printed on them."

The envelopes were postmarked from five different states and were not addressed to specific people, said Feldt.

About 90 of the envelopes were sent to Planned Parenthood clinics, according to Feldt. Twenty others were sent to independent abortion facilities, according to a statement by the National Abortion Federation.

The FBI and local law enforcement agencies were investigating who sent the envelopes.

Feldt said most of the clinics have continued to operate.

"We have learned that when facing domestic terrorism, staying the course is the best revenge," she said.

Anthrax hoaxes have been relatively common in recent years at Planned Parenthood and abortion clinics.

In 1998, letters claiming to contain anthrax were sent to eight abortion clinics in four Midwestern states. In 1999, at least 14 letters threatening anthrax were mailed to abortion centers across the nation. Those also turned out to be hoaxes.

The Associated Press contributed to this report.