bugging
chase
chasing
cloak-and-dagger work
consecutive
counterespionage
counterintelligence
dogging
electronic surveillance
espial
espionage
follow
follow-up
following
heeling
hounding
hue and cry
hunting
intelligence
intelligence work
military intelligence
observation
prosecution
pursual
pursuance
pursuing
pursuit
quest
searching
secret police
secret service
seeking
sequel
sequence
series
shadowing
spying
stakeout
stalking
succeeding
surveillance
tailing
tracking
tracking down
wiretap
wiretapping
trail
ailing
assailing
bailing
baling
curtailing
derailing
detailing
emailing
entailing
failing
flailing
grayling
hailing
impaling
inhaling
jailing
mailing
nailing
prevailing
railing
regaling
sailing
scaling
surveilling
unavailing
unfailing
unveiling
veiling
wailing
whaling
facing