(CNN) -- The bodies of two dolphins and the head of another have washed up along the Gulf Coast, raising red flags for federal investigators already looking into roughly half a dozen dolphin killings.
Excluding the latest three, the bodies of at least seven dolphins have turned up this year along a 120-mile stretch of the Gulf Coast from Louisiana to Florida. Some were shot, stabbed and mutilated.
It's unclear whether the latest three deaths are related to those killings, but investigators are exploring that possibility.
Moby Solangi, lead biologist of The Institute of Marine Mammal Studies in Gulfport, Mississippi, performed necropsies on nine of the dead marine mammals and will do the 10th on Wednesday.
"We have submitted those results to the federal enforcement officers, who are going to do additional exams or testing to further clarify the cause of death," he said.
Citing the ongoing criminal investigation, Solangi declined to elaborate on the results.
The Marine Mammal Protection Act states that anyone convicted of killing a dolphin could be sentenced to one year in prison and fined up to $100,000. The law also makes it a crime to feed dolphins in the wild.
The remains of the latest three dolphins were found in Mississippi.
A group of bird surveyors discovered an adult dolphin head on the west tip of Horn Island on Friday. On Saturday, someone found the body of a 1-year-old male dolphin on the beach in Gulfport. On Tuesday, another dolphin was found dead in Bay St. Louis; a teacher at a boarding school saw the animal and reported it.
It's not unusual to see dolphin bodies, or even parts of bodies, wash up. What is unusual is evidence of human interaction as the cause of death, as appeared to be the case in the first seven situations.
The first dolphin was found January 8 with a bullet wound on Deer Island, near Biloxi, Mississippi. In June, a bottlenose dolphin was found in Perdido Bay, Florida, near the Florida-Alabama state line, with a screwdriver stuck in its head. In September, one was found on Elmers Island, Louisiana, with a gunshot wound. On October 10, a dolphin was found on the beach in Dauphin Island, Alabama, with its fluke cut off.
The list continues.
On November 6, a dolphin was found dead with numerous cuts near Ocean Springs, Mississippi. Three days later, two more were discovered: another one with bullet wounds near Ocean Springs, and one on Ship Island, a remote barrier island off Biloxi, Mississippi, with its jaw cut off.
Authorities are still trying to determine whether a dead dolphin found November 2, in Bayou Julia, Louisiana, is linked to the other killings.
Jeff Radonski, a deputy special agent for the National Oceanic and Atmospheric Administration, which is leading the investigation, spoke last month about the deaths.
"We need to have witnesses come forward. We need public input and information," he said.