New Jersey mayors pen letter demanding action on mysterious ...

The mayors of 21 towns in New Jersey are demanding action in a letter sent to Gov. Phil Murphy over the mysterious drones that have been spotted flying overhead in recent weeks.

Since mid-November, large drones of uncertain origin have been repeatedly spotted in the sky at night over central and northern New Jersey.

The drones, which are larger than the type typically used by hobbyists, have raised concerns due to their proximity to both a military installation and President-elect Donald Trump's Bedminster golf course.

In the letter, the mayors in Morris County called for a full investigation into the drones "to determine the origin, purpose and compliance with applicable regulations."

They also demanded the "implementation of measures to prevent unauthorized or unsafe drone operations" in Morris County going forward, and "clear communication to local officials, law enforcement and residents regarding the findings of this investigation."

"While the benefits of drone technology are widely recognized when used responsibly and transparently, the lack of information and clarity regarding these operations has caused fear and frustration among our constituents," the mayors wrote. "Despite inquiries made to relevant authorities, we have yet to receive satisfactory answers about the purpose, operators, or safety protocols governing these flights."

The Federal Aviation Administration has imposed drone flight restrictions while authorities investigate.

Local police previously said there is no known threat to public safety.

"Morris County Sheriff James M. Gannon would like to inform everyone that the recent drone activity observed by many in our communities is being actively investigated. There is no advisable immediate danger to the public at this time," the Morris County Sheriff's Office said in a statement.

Last week, multiple Morris and Somerset county officials said in a joint statement they would continue to monitor the drone activity.

"County and local law enforcement agencies are aware of the recurring reports of drone activity around Morris and Somerset counties, and acknowledge the public's concern about these repeated sightings," they said.

On Monday, a spokesperson for the FBI said the agency is "doing all we can figure out what's going on."

"The public can continue to call our 800 line, or submit a tip online, we are acting on every substantive lead that we get," the FBI spokesperson said.

In an interview on Fox News Monday, New York Rep. Nicole Malliotakis said members of the public "deserve and demand answers" about the drones.

"I've also reached out to FAA, to the Department of Homeland Security and to the FBI asking for answers, and they are aware of these aircrafts. They are either unmanned aircrafts or drones and they are still trying to figure out where they are coming from and who's behind them. But following what happened with the Chinese spy balloon, that simply is not enough."

ABC News' Alex Faul contributed to this report.