(CNN) — Arizona investigators surveilling the man accused of shooting at the Democratic National Committee office in a suburb of Phoenix saw him posting political signs with bags of a white, powdery substance attached, officials said.

Investigators in Tempe began surveilling Jeffrey Michael Kelly, 60, as a suspect in three separate shootings of the DNC office after receiving tips from the public, Tempe Police Chief Kenneth McCoy said at a news conference Wednesday.

They watched Kelly placing the signs with the bags attached from late Monday into Tuesday morning, the chief said.

Kelly was arrested Tuesday and is being held on three felony counts of acts of terrorism and four other counts related to the shootings.

He is being held on a $500,000 cash bond and will be subject to electronic monitoring if he’s able to post that amount, Maricopa County Attorney Rachel Mitchell said at the news conference.

Kelly may face additional charges related to the powdery substance, McCoy said. It was not immediately known what the substance was.

Arizona is among the crucial battleground states in the upcoming election, where the rivalry between Vice President Kamala Harris and former President Donald Trump has intensified. The campaign season has already been marred by violence, including the alleged assassination attempts targeting Trump during a Pennsylvania rally and at his Florida golf course.

Three shooting incidents

Kelly is accused of shooting at the DNC office in Tempe on three occasions.

First, on September 16, just after midnight, Kelly fired more than 10 shots from a BB gun into the front doors and windows of the building, McCoy said.

A week later, on September 23, around the same time, about six shots were fired at the building, damaging the front windows and office signs, the chief said.

“This time the situation escalated to the use of real bullets,” McCoy said.

Surveillance video from the second incident showed a silver Toyota SUV leaving the parking lot shortly after the shots were fired.

Two weeks later, on October 6, shortly after midnight, a glass break alarm was triggered and responding officers found three bullet strikes in the front of the building, McCoy said.

The same silver SUV was seen in surveillance video, the chief said.

“Threats, intimidation or violence toward political officials, no matter what party they are a part of, are completely unacceptable,” Mitchell said. “These actions create fear, and they weaken the trust in our elections, and frankly, they weaken our system of democracy.”

“Our department recognizes the significance of this arrest, and we want to reassure our community that when you go to vote over the next 13 days, we are committed to keeping you safe,” McCoy said, denouncing political violence.

Officials believe weapons stockpile points to ‘potential mass casualty event’

More than 120 guns and more than 250,000 rounds of ammunition were found at Kelly’s home following his arrest, prosecutors said at Kelly’s initial court appearance Wednesday.

Investigators believe he may have been planning a mass casualty event, prosecutors said.

“Obviously … those numbers of guns as well as those numbers of rounds are certainly a factor that you would consider,” Mitchell said at the news conference.

Kelly was under suspicion for the 2022 theft of political signs, but, “I don’t believe charges were filed against him at that time,” McCoy said.

“We’re continuing our investigation to see if we can learn if there were any additional things he planned on doing,” McCoy said.

“Those who engage in political violence or try to interfere with elections or discourage people from participating in the political process will face serious legal consequences. We will aggressively pursue prosecution to protect our democracy and our democratic process,” Mitchell said.

The Arizona Democratic Party remains “undeterred and continue the work of ensuring voters in the East Valley turn out in full force now through Election Day,” a spokesperson for the group said.

Correction: An earlier version of this story incorrectly reported the first name of the suspect. His name is Jeffrey Michael Kelly.