We stay secure by staying in close communication with our neighbours. The EU provides a framework for doing this. Could a different framework work just as well? Possibly, but the EU is one that currently exists.
I'm not an expert on international security, despite all the John Le Carré novels I read. So I turn to someone who does, a former head of MI5 and a former head of MI6.
Sir John Sawers, until recently the head of MI6, has said that Brexit would mean the UK would be "unable to take part in the decisions that frame the sharing of data, which is a crucial part of counter-terrorism and counter-cyber work that we do these days. We would also lose access to the European Arrest Warrant.
The European Arrest Warrnet is like a super-extradition system that operates accross the EU. Under the scheme over 1000 people have been brought back to the UK to face sentencing.
For the sake of balance its worth saying that some eminant figures in the security world, such as Richard Walton, recent head of anti-terrorism at the Met, saying that leaving the EU would not make any difference to security. This could be the case - Like I said, I'm not an expert - but if the best that can be said is there would be 'no difference' that is not on its own an argument for LEAVE.