Two more samples taken from a waterway near Lake Michigan contained genetic material from invasive Asian carp, the U.S. Army Corps of Engineers said Friday.

The samples were drawn from Lake Calumet, on Chicago's South Side, during a second round of testing triggered by the discovery of Asian carp DNA there in May and June. The DNA was from silver carp, one of two Asian species threatening to enter the Great Lakes after migrating northward in the Mississippi River and its tributaries for decades.

Samples from a third round of testing in Lake Calumet are being analyzed, as the government's Asian carp control policy requires. If they also test positive, federal officials will decide whether rapid-response measures are needed, such as a stepped-up search for actual carp using commercial netting and electric jolts to stun fish so they can be more easily landed.

Earlier this week, the Army Corps reported that nine water samples taken this spring — including seven from Lake Calumet — had tested positive. The other two were from the Chicago River.

The samples were taken above an electric barrier about 25 miles from Chicago designed to prevent aquatic species from moving between the Great Lakes and Mississippi River basins.

Federal officials contend the barrier is working well and there's no evidence any Asian carp have passed through it. Just one live Asian carp has been found on the Lake Michigan side of the barrier.

Of nearly 5,000 water samples taken beyond the barrier, 83 have been found to contain Asian carp DNA. Critics of the government's control strategy say those numbers suggest carp are eluding the barrier and stronger action is needed.

"I don't think it's smart to burn a lot of time and effort trying to determine if the fish are there," said Joel Brammeier, president of the Alliance for the Great Lakes, a Chicago-based environmental group. "We already know they are there. We know the solutions we have are not sufficient."

Environmentalists favor placing structures in Chicago-area waterways to sever the century-old, man-made link between Lake Michigan and the Mississippi system. The Army Corps is studying that and other possible methods of preventing aquatic species from moving between the two basins, but its report is scheduled for release in 2015. Critics say that's too slow.

Five states — Michigan, Minnesota, Ohio, Pennsylvania and Wisconsin — have filed a federal lawsuit demanding quicker action. Some scientists say if the large, voracious carp establish a foothold in the Great Lakes, they could unravel the food web by gobbling plankton needed by smaller fish that feed prized sport varieties such as walleye and trout.

John Goss, who oversees the Obama administration's Asian carp policy through the White House Council on Environmental Quality, said government agencies were using the best available science and technology to monitor Chicago-area waterways for signs of the invaders.

"All the evidence tells us that our strategy is working and there is no self-sustaining Asian carp population above the electric barriers," Goss said.

Environmental DNA, or eDNA, shows only that "there is carp material in the water," he said. "At this time, eDNA doesn't tell whether the traces are from a live fish, a dead fish, bilge water or a number of other possible sources."

Fisheries experts say the fact that the overwhelming majority of samples have found no trace of Asian carp means "either no Asian carp exist in the area, or they are present in extremely low numbers and are not anywhere near a self-sustaining population," Goss said.

Brammeier said state and federal monitoring teams are doing their best with imperfect tools, but live Asian carp are notoriously hard to catch. That makes the DNA findings all the most important, he said.

"We need to stop this constant downplaying of what the DNA tells us," he said. "It's still not being taken as seriously as it ought to be."