(NewsNation) — Federal law enforcement officials warned about a potential infrastructure attack in August after a suspected white supremacist posted a file online showing the locations of power stations across North America, according to an internal memo obtained by NewsNation.

The correspondence suggests federal authorities were concerned about threats to American infrastructure posed by extremists just months before the substation attacks in Moore County, North Carolina, that left nearly 45,000 Duke Energy customers without power.

Officials called the recent power station shootings “targeted” but have not determined a motive.

NewsNation located the database authorities believe the online poster — who is described as a “suspected racially or ethnically motivated white supremacist” — shared with others. It uses public data from the U.S. Department of Homeland Security and shows the exact coordinates of more than 75,000 substations across the U.S. and its territories.

The database included multiple substations that have been targeted recently in North Carolina and, as NewsNation uncovered Wednesday, in Florida.

Addressing Security Threats

Experts say these physical security threats are not something most utility companies are used to preparing for.

“The power company is focused on safety, not necessarily security, not necessarily protecting against someone with the intent to go into a substation to disable that substation,” 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).

In September, Duke Energy power stations in Florida experienced at least six “substation intrusion events,” according to an incident report obtained by NewsNation.

In an email to NewsNation, Duke Energy said it is “continuously assessing” and “evolving” its security measures to protect critical infrastructure.

Elsewhere, the electrical grid has been attacked at least four times in Oregon and western Washington since late November, according to Oregon Public Broadcasting (OPB). Attackers used firearms in at least some of the incidents in each state, OPB found.

While authorities work to pin down motives in all of the recent attacks, those who track extremism are not surprised.

“We see attacks on various types of energy infrastructure being glorified across the ideological spectrum,” said Brian Levin, the director of the Center for the Study of Hate and Extremism at California State University, San Bernardino.

Levin said disrupting infrastructure can be an attractive option for those looking to cause chaos because it can have a great effect with minimal cost to the attacker.

In April 2013, gunmen in Coyote, California, fired on 17 electrical transformers causing $15 million worth of damage. The perpetrators were never found.

In response, Pacific Gas and Electric Company (PG&E) committed $100 million to secure its highest-priority facilities.

It remains to be seen what additional long-term security measures, if any, power companies will put in place after the most recent incidents.