Rocky Mount, N.C. — School officials in Nash County can open mail only after students go home for the day because of three powder-filled packages recently sent to area schools.

A principal at Pope Elementary School opened a letter containing a white powder on Wednesday, becoming the third Rocky Mount school to receive such a package in the past 10 days.

On Tuesday, Parker Middle School went into lockdown and all students were kept in classrooms as police investigated a powder-filled envelope mailed there. Another letter with a powdery substance showed up at Baskerville Elementary School on Jan. 29.

No one was injured in any of the incidents. Initial tests on the three powders found that they were nontoxic.

All three schools are less than a mile apart, and authorities said none has received threats before.

"Anything that involves the safety of our staff and our children is of great concern. It's serious," said Leorita Hankerson, assistant superintendent of Nash-Rocky Mount Schools.

The schools were calling parents to inform them of the situation.

Rocky Mount police said they believe the three letters are related, and they are working with the FBI to find the sender, who could face state charges of communicating threats and federal charges of mail tampering.

Last year, lawmakers voted to make it a felony for a person to report a false threat of mass violence on educational property. It's unclear whether that law would apply in the Rocky Mount cases.

Authorities wouldn't discuss details of the letters, but they have instructed school staff not to open any mail until students leave campus.

"Our principals were shown a copy of the envelopes that were received so that they would be on the lookout for specifics," Hankerson said.

Powder-filled packages have been associated with anthrax since 2001. Five people died from the substance when anthrax-laced letters were mailed to lawmakers and news media just weeks after the Sept. 11 terrorist attacks.