A series of apparently random shootings along Californian main roads are giving drivers the jitters.
At least seven attacks in the past six weeks have resulted in five deaths and several injuries.
Jammed commuter corridors have been brought to a halt while authorities searched for clues.
But officials said the shootings were unconnected.
"There is no trend," said a Highway Patrol spokesman. "The public tends to think this is an increase; it's just random incidents."
Since the end of February a 54-year-old woman died after she was shot in the head while driving on the 10 Freeway in eastern Los Angeles County; a 26-year-old man died after a shooting on the 101 Freeway following an argument on Hollywood Boulevard; and two teenage boys died after they were shot on a state highway in San Diego County.
Another death was being investigated as a possible suicide.
Authorities also were investigating an apparent airgun attack that shattered rear windows of six vehicles on the 10 Freeway in eastern Los Angeles County.
With witnesses hard to find, no arrests have been made in any of the cases. Some experts believe such high-profile incidents can spawn copycat attacks.
"When you see an event like this happen, it does create a suggestion in the minds of others to do the same thing," said Lawrence Palinkas from the University of Southern California who researches the psychological effects of shootings and other traumas.
Police can struggle when investigating the attacks. Officer Jackie Bezart, a spokeswoman for the Long Beach Police Department, said potential witnesses often speed by shooting scenes, unaware that an attack occurred.
"For the millions of miles that are travelled on California freeways, the number of shootings is few and far between," said the Highway Patrol spokesman.
Freeway shootings gained widespread notoriety in 1987, when a string of about 50 unrelated shootings in Southern California left at least five people dead and more than a dozen injured.
The attacks were blamed in part on increased anger and frustration over rising traffic volume and roadway construction.
In 1998, three attacks on women drivers and several other shootings left at least three dead.
In the summer of 2005, four people were killed in freeway shootings in the Los Angeles area.
Police arrested two men in connection with one of the killings but later dropped charges after they were unable to find a key witness.