The job of the intelligence officer is to identify those
strands that are worth pursuing and then to pursue them until either
they are resolved, or they start to look flaky and not worth pursuing,
or there is nothing more that can usefully be done. It is a
risk-management process. The number of potential leads that can be
followed is virtually infinite. On the other hand, covert investigation
is extremely resource-intensive and impinges1 on the human rights of the subject. The threshold2
for such investigations is therefore high and the number of
investigations necessarily limited. Consequently many potential leads
have to be discounted. Decisions on which leads to pursue are vital, but
are also complex and rich in judgement.
1 impinge - to have an effect on (something) often causing problems by limiting it in some way.
2 threshold - a point or level at which something begins or starts to take effect.
Michael Herman. Internet: <http://www.csis-scrs.gc.ca/eng/comment/com83_e.html> (with adaptations).
Based on the text, it can be deduced that
"On the other hand" means out of hand