(CNN) — Five Missouri state senators, all Democrats, found stickers resembling rifle crosshairs on the doors to their capitol offices Tuesday as the legislature debated health care reform, several senators said.

“This is a very open act,” said Sen. Maria Chappell-Nadal, who said she was taking it as a serious threat.

Police presence in the hallways of the capitol in Jefferson City was stepped up after the discovery, Chappell-Nadal said.

Capitol police and the Missouri State Highway Patrol have taken the brass nameplates where the small orange stickers were placed, she said. A police spokesman would not comment to CNN.

Sen. Robin Wright-Jones, one of four African-American female senators whose doors had the stickers, said she was not intimidated. “It was a cowardly act,” Wright-Jones said.

About 90 minutes after one sticker was removed from Chappell-Nadal’s door, another larger crosshair sticker appeared, Wright-Jones said.

Sen. Jolie Justus, who said a lot of people walk by her door, found her sticker during the lunch break between a debate on federal health care reform.

Two other legislators who found stickers on their doors — Sen. Shalonn “Kiki” Curls and Sen. Victor Callahan — did not immediately respond to CNN requests for comment.

— CNN’s Kara Devlin contributed to this report.