An attorney for Mr Monahan, Kurt Mausert, told CNN he was unaware that the bullets he fired had hit someone and had no idea why police were on his property.
The arrival of the vehicles "certainly caused some level of alarm to an elderly gentleman who had an elderly wife", he told the New York Times, adding that his client "sincerely regrets this tragedy".
"When there's a tragedy and a victim, everyone wants a villain," he said. "But sometimes they're just tragedies and victims and there are no villains. And this is one of those times."