Five states are asking a federal judge in Chicago to take emergency action to close two shipping locks and install barriers to prevent Asian carp from overrunning the Great Lakes via a "carp highway."

At the first hearing in the case Monday, Judge Robert M. Dow Jr. showed no signs of rushing into a decision. He scheduled Sept. 7 and 8 to hear expert testimony in the case, including from scientists about the environmental DNA testing that has found genetic material from Asian carp in Illinois waterways near Lake Michigan.

The judge's questions reflected awareness of the DNA test's limits.

"Could it have been from something that ate a fish?" the judge asked about carp DNA found in water samples. Michigan assistant attorney general Robert Reichel acknowledged a bird that ate an Asian carp could excrete carp DNA into the water. The states' experts believe it's more likely that the findings show the recent presence of carp, Reichel told the judge.

The judge also asked about a single 20-pound carp discovered in June, the first to be found in a Chicago waterway above the electric barrier system. The judge asked whether scientists could pinpoint how it got there.