The Navy sub base in Groton was on lockdown briefly on Wednesday morning because of a security alert, according to Groton police.
It was over as of 11 a.m.
No one was allowed onto or off the base during the lockdown.
Officials from the sub base would not elaborate on what caused the security alert,
The base is the home port for about 16 nuclear-powered attack submarines.
More information will be posted once it becomes available.