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(NewsNation) — Just days after what officials are calling a “targeted” attack at two Duke Energy substations in North Carolina, NewsNation has obtained federal documents showing evidence of at least six other “intrusions” at Duke Energy substations in Florida.

In September, Duke Energy Florida experienced at least half a dozen “substation intrusion events,” according to an incident report obtained by NewsNation.

On Sept. 21, an intruder “forced entry” into the Zephyrhills North substation in Pasco County, manually tripping equipment that caused an outage lasting nine minutes, according to a report filed with the U.S. Department of Energy.

One day later, someone “forced entry” at Duke’s East Clearwater substation in Pinellas County, again manually tripping equipment that caused an outage lasting two minutes.

The two substations are about 50 miles apart and both incidents took place in the early morning hours between 2 a.m. and 4 a.m.

Experts say the threats to infrastructure are nothing new but appear to have become more common recently.

“It’s definitely not a new type of threat but I think we’re seeing a level of intent to cause damage that is higher than we’ve probably seen in the past,” said Todd Keil, an associate managing director for security risk management at Kroll, who previously worked with the U.S. Department of Homeland Security (DHS).

The report filed by Duke Energy Florida said the two latest incidents followed four other “similar substation intrusion events,” all of which occurred in the state during the month of September.

Earlier in the month, an intruder entered Duke’s Bay Ridge substation in two separate incidents on Sept. 10 and Sept. 13. The Orange Blossom substation was targeted on Sept.18, as was the Zephyrhills substation on Sept. 21.

Duke Energy issued the following statement after NewsNation asked for additional information about the incidents in Florida:

We can’t comment on any ongoing legal proceedings or investigations. However, given the nature and scale of our operations, we – alongside federal, state and local law enforcement and security officials and industry partners – are continuously assessing and evolving our measures to protect our critical infrastructure.  That partnership includes helping bring anyone, who damages our system, to justice. 

Duke Energy

Federal law enforcement suspects the people behind the Florida intrusions likely have inside knowledge of the grid and understand how to power down equipment without causing damage, according to a memo obtained by NewsNation.

“The fact that someone has potentially identified a critical substation and then has knowledge of those critical pieces of equipment inside that substation leads me to believe that they’re dealing with people who have inside knowledge,” the memo read.

Duke Energy Florida said it is actively investigating the incidents with law enforcement and has installed video surveillance equipment and posted guards at substations in the area, according to the incident report dated Sept. 22.