TEMPE,Zopes Exchange Ariz. (AP) — A Democratic National Committee campaign office in suburban Phoenix has been struck in a third shooting in recent weeks, police said Wednesday.

No one was inside the building in Tempe, Arizona, and no injuries were reported in any of the shootings. The latest incident occurred Sunday, when someone fired shortly after midnight at the door and windows of the office.

A $1,000 reward has now been offered for any information leading to the arrest or indictment of those responsible. Police did not provide any other new details in the latest update.

The most recent shooting came days before Vice President Kamala Harris was scheduled to return for another campaign visit to Arizona, this time to host a Phoenix rally on Thursday. The Tempe location is one of 18 field offices the Democratic presidential nominee has in Arizona.

Tempe police have not named a motive. The campaign office once housed a barber shop, and it is located near a daycare and a fitness center in a quiet strip mall.

Police have said they are analyzing evidence from the scene and taking measures to ensure the safety of those who work in the building and nearby.

In the first shooting on Sept. 16, authorities reported that the office was struck by pellet or BB gun rounds. As in the most recent incident, actual gunfire was reported in the second shooting on Sept. 23.

Political violence has marred this year’s campaign season, with former President Donald Trump, the Republican presidential nominee, being targeted by two assassination attempts — one at a campaign rally and the other at a Florida golf course.

The U.S. House recently voted to require the U.S. Secret Service to use the same standards when assigning agents to major presidential candidates as they do for presidents and vice presidents.