But investigators acknowledge that a number of suspects are still at large.

Investigators acknowledge that it may prove difficult to construct a case after so many years.

But investigators acknowledge that they still have no idea who is behind the tainted letters.

Yet several investigators who had earlier been skeptical about the trace evidence acknowledged that the revelation turns up the pressure to find the cause.

Investigators acknowledged later that the man had been interviewed in the presence of his lawyer, but they declined to say what was discussed.

Investigators called it a revised version of an earlier sketch, but also acknowledged the possibility that two men might have been involved in the crime.

The investigators acknowledge that each of these theories has internal inconsistencies they cannot explain.

The investigators acknowledge that they have no evidence pointing to a mechanical malfunction.

Investigators acknowledge that none of the theories have yet gained pre-eminence.

Investigators backed off that finding under relentless criticism and acknowledged the possibility of an accident.