The number of attempted attacks on the U.S. power system has risen to between 3,000 and 4,000 per month, according to the former head of the nation’s largest electricity operator.

PJM Interconnection’s former CEO, Terry Boston, discussed the challenges of the electric grid at President Trump’s National Infrastructure Advisory Council quarterly meeting Tuesday, led by the Department of Homeland Security. “At PJM, we had 3 to 4,000 attempts per month at our firewalls to get into our system,” he said at the meeting.