(CNN) -- Amanda Fuller grips the wheel tightly as she drives through Kansas City after a series of shootings targeting drivers in the Missouri city over the past month.
"Every time I pass a driver, we are both looking at one another, as we are all on the defense with the recent shootings," Fuller said in a Facebook post on the city Police Department's website. She said she drives the Interstate 435-470 corridor, where most of the shootings have occurred.
"It saddens me and it is a shame."
Local and federal authorities, including the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives, are trying to find out who is behind as many as 20 shootings over the past month. The shootings all occurred on major highways and roads around the city between March 8 and April 6, police said. No new shootings have been reported since Sunday night, but police say they expect the numbers to change as they investigate more incidents and rule out others.
A U.S. law enforcement official briefed on the case said Thursday that investigators are still trying to figure out if the shootings are related and, if so, how many might be connected. The official said federal agents used dogs on I-435 Wednesday to search for shell casings that might tie the cases together. They are not saying whether they found anything, or if the same caliber of firearm was used in some or all of the cases.
The official told CNN that there are similarities among the cases. The attacks seem to be concentrated in certain areas. Almost all of the shootings occurred on the freeway near entrance and exit ramps.
One of the victims, who asked to remain anonymous, told CNN affiliate KCTV on Tuesday that three bullets went through his driver's-side door Sunday night as he was heading east on I-435 near I-470.
A bullet struck him in the calf. "We just don't know if they're doing it until they finally kill someone, or is it just for fun? We just want to know why," the victim's wife said.
"It's unaimed, not targeted," motorist Ashley Quinn told KCTV. "You don't know who they are shooting at. It could be anybody."
Police are increasing their presence in the areas where the shootings have occurred, and some drivers said they were taking secondary roads, especially after nightfall.
Investigators are asking drivers to remain vigilant and to report any suspicious activity immediately. Many residents are confident police will figure out who is taking aim at drivers.
"It's not going to change my life." Kansas City BP service manager Chris Wilson said. "I still feel safe on the road. ... The police will probably catch whoever is behind these shootings."
Authorities are offering a $7,000 reward for information leading to an arrest in the case.
CNN's Evan Perez and Carma Hassan contributed to this report.