By last month, state transportation officials in Missouri said they had seen enough.

Federal highway officials had long insisted that guardrails throughout the state were safe. But some guardrail heads had apparently malfunctioned, in essence turning the rails into spears when cars hit them and injuring people instead of cushioning the blow, Missouri officials said.

“The device is not always performing as it is designed and intended,” a Missouri transportation official wrote of the problematic rail heads in an internal communication.

Because of its safety concerns, Missouri banned further installation of the rail heads on Sept. 24. It joined Nevada, which prohibited further purchases in January, and was followed six days later by Massachusetts. Lawsuits say the guardrails were to blame for five deaths, and many more injuries, in at least 14 accidents nationwide.

The Federal Highway Administration, the agency charged with ensuring the safety of the nation’s roads, continues to say that the guardrails, installed on highways in almost every state, meet crash-test criteria. And the Texas-based manufacturer, Trinity Industries, a large supplier of guardrails nationwide, also denies there is a problem.