Iranian-linked hackers have targeted a broad range of U.S. critical infrastructure in recent weeks, including water and wastewater facilities, telecommunications networks, energy systems and other infrastructure, NBC News reports. The attempted attacks focused on internet-connected automated and industrial control systems and have so far been unsuccessful.

The new activity follows a major wave of cyberattacks in July in which more than 100 U.S. water and wastewater systems across 12 states were targeted.