Spies from other countries have hacked into the United State’s electricity grid, leaving traces of their activity and raising concerns over the security of the U.S. energy infrastructure to cyberattacks.

The Wall Street Journal on Wednesday published a report saying that spies sought ways to navigate and control the power grid as well the water and sewage infrastructure. It’s part of a rising number of intrusions, the article said, quoting former and current national security officials.

The intruders don’t appear to have done any damage to date but did leave behind software that could disrupt the system.

“The Chinese have attempted to map our infrastructure, such as the electrical grid,” a senior intelligence official told the Journal. “So have the Russians.”

There have long been concerns over securing the power grid and other infrastructure. Those security issues are mounting as utilities use more Internet-based communications and software to control the grid through smart-grid technology.

CNET: Report: Spies hacked into U.S. electricity grid