U.S. officials say the Department of Homeland Security is to lower the country's terrorist threat level for mass transit.

The level had been raised last month from "elevated" to "high" for U.S. buses, subways and trains in response to the July 7 suicide bombings in London that killed 56 people, including four attackers.

The alert level is expected to be lowered back to "elevated" after rush hour Friday.

In raising the threat level last month, Homeland Security chief Michael Chertoff said the move was precautionary, and that U.S. officials had no credible information of an imminent attack in the United States.

Some information for this story provided by AP and Reuters.