line of inquiry
line of questioning

This information opened up a whole new line of inquiry.

However, he said it was "not a significant line of inquiry".

He had no idea if this would be a useful line of inquiry.

As a result police now say they are following a definite line of inquiry.

This turned out to be the wrong line of inquiry.

This line of inquiry did not lead to any new information about the woman's identity.

Several small research projects continue this line of inquiry in Europe.

Now there would be two lines of inquiry running tomorrow.

So he had all the names down: would start the new line of inquiry at once.

I have my usual lines of inquiry out, but nothing to report at present.