1.   Rarely does the refuse offer hints about who might have dropped it off, making perpetrators impossible to track down.

2.   The company said Friday that it had contacted the FBI to help track the perpetrators.

3.   Defense Secretary William Cohen said the perpetrators would be tracked down.

4.   The Lockerbie trial illustrates the difficulties of finding evidence that stands up to judicial scrutiny and of tracking the perpetrators to their source.

5.   The Lockerbie trial illustrated the difficulties of finding evidence that would stand up to judicial scrutiny, and of tracking the perpetrators to their source.

6.   The police have been accused of sluggishness in tracking down perpetrators of acid attacks and some assailants have gone free on bail soon after their arrest.