"It's hard to tell what we saw there," said Michael Bicay, science director at NASA's Ames Research Center in Mountain View, Calif.
Debris from the impact could have flown horizontally, or perhaps didn't clear the crater's rim, lead mission scientist Anthony Colaprete told reporters.
"Some luck has to come to get the ejecta to fly in the direction you want it to fly," he said.
"I'm not convinced we haven't seen the ejecta," Colaprete added. "We just have to go back with a finer tooth comb."