彼女はわめき、警官たちに向かってナイフを振り回して脅すようなしぐさをしました。

、。

She was screaming, swinging and thrusting a knife at the police officers in a threatening manner.

Literal

She [topic-は] screaming, police-officers [toward-に] toward knife [object-を] brandishing threatening [like-ような] gestures did.

わめき ('screaming, shrieking') is rougher than 叫ぶ — implies uncontrolled, desperate noise. 振り回して uses the て-form to chain actions. 脅すようなしぐさ ('threatening-like gestures') uses ~ような for comparison and しぐさ ('gesture, mannerism') for the physical behavior. The polite ました form with violent content mirrors formal news reporting style.