Java Executors: how can I stop submitted tasks?

I have submitted a task using executors and I need it to stop after some time (e.g. 5 minutes). I have tried doing like this:

   for (Future<?> fut : e.invokeAll(tasks, 300, TimeUnit.SECONDS)) {
         try {
             fut.get(); 
         } catch (CancellationException ex) {
             fut.cancel(true);   
             tasks.clear();
         } catch(ExecutionException ex){
             ex.printStackTrace(); //FIXME: gestita con printstack       
         }
   }

But I always get an error: I have a shared Vector that needs to be modified by the tasks and then read by a thread, and even if I stop all the task, if the timeout occurs I get:

Exception in thread "Thread-1" java.util.ConcurrentModificationException

Is there something wrong? How can I stop the tasks submitted that are still working after 5 minutes?