Select Subset in Linq

I am trying to write a linq query which will exclude any records that have a child record with a certain integer ID.

The class I am querying against looks like:

public class Group {
     public ICollection<Item> { get; set; } // This is the child collection
}

public class Item {
     public int Id { get; set; }
}

My repository query method is:

public ICollection<Group> Get(int itemId) {

      return from c in Set.... // Set is an EF collection of all Groups
}

I want to return all Groups that do not have an Item in their Items collection with the Id equal to the itemId passed to the method.

Not sure how to write this most efficiently in Linq.