US Capitol Police were investigating a possible threat against Todd Akin, the congressman who triggered an uproar when he said a woman's body can prevent pregnancy in cases of rape.
"The US Capitol Police are currently working with the FBI on a reported threat against Congressman Akin," spokesperson Lieutenant Kimberly Schneider told AFP.
"This is an active, open investigation."
She declined to discuss the security of specific lawmakers, citing standard protocol.
The Federal Bureau of Investigation's Washington field office confirmed it was aware of the alleged threats.
The Republican establishment, including party leaders, have pressured Akin to bow out of his race to unseat incumbent Democratic Senator Claire McCaskill in November's election due to his shock comments on Sunday about "legitimate rape" and his opposition to abortion in cases of rape.