Police found ammunition when they raided two addresses in the same street.
Officers used a battering ram to force open the doors of homes in Curtis Close, Mill End, this morning.
Once inside, they seized a quantity of ammunition.
The ammunition has been sent off for examination.
Closure orders on the addresses have been put in place by Three Rivers District Council, to stop anyone but the homeowner getting in.
Full closure orders are being sought.
Teams from across Watford and Three Rivers were involved in the raids.
Three Rivers Chief Inspector Nick Lillitou said: “Today’s operation shows that we will act on intelligence to deal with those who are suspected of being involved in criminality.
“I hope it sends a clear message that we will use any means available to us to bring people to justice and improve the quality of life for residents who may have been detrimentally affected by this kind of behaviour.”
• A 24-year-old man from Luton and a 19-year-old woman from Rickmansworth were arrested on suspicion of possession of ammunition without a certificate. They remain in police custody.
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