US authorities say preliminary tests on a letter addressed to President Barack Obama are positive for the deadly poison ricin.
In a statement, the Federal Bureau of Investigation said the investigation into the letter sent to President Obama and another sent to Senator Roger Wicker was ongoing.
The FBI said there was "no indication of a connection" to the bomb attacks at the Boston Marathon that killed three people and injured more than 170 others.
Additional tests would be carried out over the next 24 to 48 hours to confirm the presence of ricin.
The US Secret Service said the letter had been intercepted at a mail screening facility outside the White House on Tuesday, the same day authorities said a letter was sent to Senator Wicker that also showed traces of ricin.
Secret Service spokesman Edwin Donovan said the agency, which protects the president and his family, was working closely with the US Capitol Police and the FBI to trace the origins of the letter.
At the US Capitol, sections of two Senate office buildings were briefly cordoned off amid reports of a suspicious package, AFP reports.
Capitol Police later said results of tests conducted at the Hart Senate office building were negative and the closed-off areas were reopened.
US Capitol Police confirmed one man was being questioned but had not been detained.
Senator Carl Levin issued a statement on Wednesday saying one of his staff members had discovered a "suspicious-looking letter" at a regional office in Michigan and handed it over to authorities for further investigation.
The episodes recalled a series of letters laced with anthrax that were sent to politicians and journalists following the September 11 attacks in 2001. The anthrax killed five people and made 17 others ill.
Congressional mail has been screened off-site since the 2001 incident.
Ricin, when inhaled, can cause respiratory problems. Ingested orally, the protein is lethal in even minuscule quantities.
