(CNN) -- With each new blaze that erupts in Virginia, the urgency to arrest a "group" of arsonists grows.
At least 71 fires have occurred in Accomack County since November, the latest reported Monday.
And in each case, whoever is responsible slips away into the night, unnoticed.
Most of the blazes are discovered between 6 p.m. and midnight, many about two hours after they are lit, according to authorities.
"It's very worrisome," said Corinne Geller, a police spokeswoman. "We've been very fortunate that no one has been injured or killed."
She said police are confident that they are looking for a group working together.
Authorities have their theories on what might be motivating the group, but they are not ready to go public with those, she said.
"Some of the targeted structures have been in very remote, random locations throughout this rural county," said Capt. Timothy A. Reibel, commander of the Bureau of Criminal Investigation Chesapeake Field Office.
"This not only makes the fires more difficult to detect, but also enables the criminal to come and go with minimal detection."
Reibel's comments were in a news release that offered a $25,000 reward for information leading to an arrest and conviction.
"If you see something suspicious, call us. If you see someone or something that looks out-of-place, call us. The public is the eyes and ears of its community," he said.
Police boosted uniformed patrols and surveillance in the county and brought in investigators from across the state.
"We've got some strong leads, and we're moving forward," said Geller, the police spokeswoman.
State authorities have received 200 tips since setting up a phone number for information in February.