Researchers think a mysterious die-off of seals along the Maine coast could be linked to chemical pollution, the Portland Press Herald reported Sunday.

More than 400 dead or stranded seals have washed up on the Maine coast so far this year, more than in any of the past seven years, according to National Oceanic and Atmospheric Administration (NOAA) statistics.

The immediate cause of the die-off is not yet known, but marine biologist Susan Shaw told the Portland Press Herald it could be related to toxic chemicals like polychlorinated biphenyl (PCB), which are found in high quantities in Maine harbor seals and make it harder for marine mammals to fight off disease.

"We find this in young animals. They are immune-suppressed from birth," she said. "When some pathogen comes along like this, they are very susceptible to becoming very sick and dying very quickly."

[Watch seals being rescued in Ireland.]

More than 130 seal deaths have been reported in Maine in the past 30 days, and the die-off is spreading south, with 26 dead seals reported in New Hampshire and Massachusetts in July and 25 in August so far, NBC Boston reported Aug. 17.

"It's significantly more than we should expect to see for those states this time of year," NOAA greater Atlantic regional spokesperson Jennifer Goebel said. "We aren't seeing any external injuries. There have been reports of coughing, sneezing, and lung issues."

NOAA researchers are testing to see if Avian flu or phocine distemper might be the culprits. Initial results should be ready this week.

Out of concerns the die-off might be caused by an infectious disease, NOAA has asked the non-profit Marine Mammals of Maine (MMoME) to stop taking in live, stranded seals out of fear of infecting healthy animals.

Before the order, the group was rehabilitating some of the still-living seals at their Harpswell, Maine center.

MMoME has been on the front lines of the mysterious, lethal event, responding to 100 dead seals on the beaches south of Portland and in Casco Bay just last week, MMoME Executive Director Lynda Doughty told the Portland Press Herald.

Doughty said that spring and summer are typically dangerous times for seals, since young animals are trying to survive on their own for the first time, adults are molting and they are more likely to be struck by boats. But Maine usually sees around 38 dead seals in August, Goebel told NBC Boston, not more than 100.

"Having a hundred since Sunday is not normal," Doughty told the Portland Press Herald. "For us, July seemed to be pretty steady. Once August hit our numbers kept rising, rising, rising."