(USA TODAY) -- The regional offices of two U.S. senators have been evacuated and closed after suspicious packages were received there.

The Saginaw, Mich., office of Sen. Carl Levin, D-Michigan, received a letter Wednesday that the staff believed to be of a suspicious nature, Levin spokeswoman Tara Andringa said.

Also, Wednesday, the Phoenix office of Sen. Jeff Flake, R-Ariz., was evacuated after authorities received reports of a suspicious package. It is unknown what made the package suspicious, Phoenix police said.

The first floor of the building was evacuated as a precaution, authorities said, and the ventilation system has been turned off.

The closing of the offices come as authorities in Washington have reported suspicious packages being sent to senators and President Barack Obama. On Tuesday, officials said a letter addressed to Mississippi Sen. Roger Wicker tested positive for ricin, a poison. The letter to the president also tested positive for ricin.

Both letters were detected at an offsite mail facility and never reached the White House or the Capitol complex.

Officials have been on high alert since Monday's bombing at the Boston Marathon which resulted in three deaths and more than 100 casualties.

Saginaw authorities have taken the letter received by Levin's office and are investigating.

"Earlier today, a staffer at my Saginaw regional office received a suspicious-looking letter. The letter was not opened, and the staffer followed the proper protocols for the situation, including alerting the authorities, who are now investigating," Levin said in a statement. "We do not know yet if the mail presented a threat. I'm grateful for my staff's quick response and for government personnel at all levels who are responding."

Andringa could not say what made the letter seem suspicious, but said it was not opened by staff. Phil Ludos, Saginaw's assistant city manager for public safety, said city hazardous materials teams evacuated the building -- which also included the Saginaw Chamber of Commerce and other offices. An FBI team from Detroit is on its way to take control of the scene.

The FBI confirmed it was investigating.

Ludos said the staffer apparently considered the letter suspicious because of an alert sent out from Washington on Tuesday after news of the letter to Wicker's office was made public. Ludos said the letter to Levin's office did not have a return address -- which was also true of the letter to Wicker's office -- but he was unsure whether there were other reasons for it to be considered suspicious.

Ludos said the letter remained in the office while the FBI team was en route to handle it.

"It's basically the FBI's scene now," he said.

Keith Moore of the U.S. Postal Inspection Service, confirmed that inspectors were at Flake's office in Phoenix.

"We do have inspectors on the scene investigating right now. I don't have any details."

Manuel Johnson of the FBI said the bureau is aware of reports that Flake's office received a suspicious package or letter, and is checking it out.

A source in Flake's office said the people who opened the suspicious package were taken to the hospital as a precaution but that no one is ill.

Brian Rogers, a spokesman for Arizona Sen. John McCain, said they were contacted by officials with the the Bureau of Alcohol, Tobacco, Firearms and Explosives because of the incident across the street at Flake's office but as far as he knows, McCain's Phoenix office didn't receive anything suspicious.

Levin, the chairman of the Senate Armed Services Committee, announced last month that he would retire at the end of his current 6-year term.

Contributing: Todd Spangler, Detroit Free Press; Jane Lednovich, Dennis Wagner and Dan Nowicki, The Arizona Republic.

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