A number of the mammals have washed ashore with bullet wounds, hacked-off fins and in one case a missing jaw.
One dolphin was found off Alabama with a screwdriver in its head over the summer, while another had its tail cut off, but survived.
Dolphins are protected by the 1972 Marine Mammal Protection Act.
Under the law, harming a dolphin can lead to fines of up to $10,000 and a prison term of one year.
The National Oceanic and Atmospheric Administration (Noaa) has appealed for people along the US Gulf Coast to watch out for dead or injured dolphins.
"There have been some obviously intentional cases," said Erin Fougeres, of Noaa.
She said some mutilations might have happened after the animals died, but that scientists would need to perform necropsies to learn more.
The Associated Press reports that three dolphins have been shot in Louisiana since 2011, and that three others have been found with fatal bullet wounds this year in Mississippi, the most recent one last week.
Moby Solangi, executive director of the Institute for Marine Mammal Studies in Gulfport, Mississippi, said: "They [dolphins] come close to the boats, so if you're out there, you'll see them riding the bows.
"And their curiosity and friendship brings them so close that they become targets and that's the unfortunate thing."
The attacks follow two bad years for the Gulf dolphin population: in 2010, a number of the mammals stranded themselves along the coast, possibly due to a cold winter.
The Deepwater Horizon fire and oil spill later that year contributed further to dolphin deaths. Scientists have recorded more than 700 such fatalities since February 2010.
Some have suggested a few angry fisherman might be responsible for the mutilations, given dolphins sometimes steal bait used by the boats.
US wildlife officials are investigating a spate of killings and mutilations of dolphins along the Gulf Coast over the past year.