A series of assaults have taken place against men in downtown Wilmington, North Carolina since January 2018
CNN  — 

Eleven men, including several members of the military, have been assaulted near downtown bars in Wilmington, North Carolina, over the course of nearly two years, authorities said.

Police believe the alleged assaults are connected, but solving them has been difficult because many victims don’t recall the details, Wilmington Police Chief Ralph M. Evangelous said in an interview on Thursday.

The victims were assaulted around the time the bars closed, Wilmington police said. Some victims were robbed. Four victims may have been sexually assaulted, and seven victims were military personnel, police said.

Since the summer, detectives in Wilmington have combed through old police reports and re-interviewed victims to piece together the investigation. Wilmington police have identified several common factors: the victims were drinking at bars, they left alone and woke up alone, Evangelous said. Many victims did not immediately report the alleged incidents.

“They have no memory of where it occurred. … It could’ve been in a car, it could’ve been in an apartment, we don’t know,” Evangelous said. “So, that’s been a struggle for us.”

He said one to two victims have a “fuzzy memory” of the alleged incidents.

The men, who are mostly in their 20s and 30s, were victims of “probably a crime of opportunity.”

Minor assaults are not unusual in the downtown bar district on the weekends, when as many as 2,000 people flood the streets when the bars close, the police chief said. “What was unusual is the M.O., which I cannot get into right now because it’s critical to the investigation,” Evangelous said.

He said police “believe that there is a connection here to most of these, if not all of them.” Evangelous declined to reveal the connection.

Investigators believe the assaults could have started as far back as January 2018. There was a four-month lag in time between some assaults, police said. Some of the initial reports came in as misdemeanor crimes, such as breaking and entering, with limited details, which made the investigation challenging, authorities said.

Investigators made the connection about a month ago because the crimes escalated, CNN affiliate WCET reported.

Mark Mueller, the co-owner of Waterline Brewing Co. in downtown, told CNN affiliate WWAY the brewery usually takes steps to keep its employees and patrons safe.

“You never want to have one person closing. You never want to have someone walk to their car by themselves. You never want to have someone walking home alone,” Mueller said.

Wilmington police say they have increased patrols in the downtown area. Authorities also encourage customers not to leave their drinks unattended and to travel in groups.

Evangelous said police have sought help from the military, including the Naval Criminal Investigative Service, and various task forces.

“We just ask for patience,” Evangelous said. “I know people are frustrated, we’re frustrated, but we’re putting a lot of time and a lot of effort, and a lot of partners are working with us trying to get this thing pulled together.”

CNN’s Amanda Watts and Jamiel Lynch contributed to this report.