Just before a tornado descended on St. Louis with a roar — killing five people and injuring dozens during its sweep through the city on Friday — there was a silence where there should not have been.

There was no wailing warning from the city. No high-pitched alarm. Nothing to warn the city’s residents and send them scrambling to their basements or bathtubs. Only wind.

The city’s sirens to warn people of a tornado threat were never activated by the City Emergency Management Agency, and a backup to activate the mechanism that is operated by the Fire Department was broken.

Mayor Cara Spencer has placed the city’s emergency manager, Sarah Russell, on paid administrative leave while an investigation is conducted into a series of failures, Ms. Spencer’s office said in a statement issued on Tuesday. The mayor’s office also said that it had changed the protocol for activating the warning system as a result of what had happened.

The city’s emergency management agency “exists, in large part, to alert the public to dangers caused by severe weather, and the office failed to do that in the most horrific and deadly storm our city has seen in my lifetime,” Ms. Spencer said in her statement.

Ms. Russell could not immediately be reached for comment on Wednesday.

City officials confirmed that one of five people killed in Friday’s storm was outside when the tornado ripped through St. Louis. About 40 people were injured in the storm, but city officials did not know how many of them were outdoors when they were hurt.