diff --git a/sqs_workers/memory_sqs.py b/sqs_workers/memory_sqs.py
index e1a54ae..bb8bdb0 100644
--- a/sqs_workers/memory_sqs.py
+++ b/sqs_workers/memory_sqs.py
@@ -107,7 +107,7 @@ class MemoryQueue:
     name: str = attr.ib()
     attributes: Dict[str, Dict[str, str]] = attr.ib()
     messages: List["MemoryMessage"] = attr.ib(factory=list)
-    in_flight: List["MemoryMessage"] = attr.ib(factory=list)
+    in_flight: Dict[str, "MemoryMessage"] = attr.ib(factory=dict)
 
     def __attrs_post_init__(self):
         self.attributes["QueueArn"] = self.name
@@ -147,7 +147,8 @@ def receive_messages(self, WaitTimeSeconds="0", MaxNumberOfMessages="10", **kwar
             else:
                 ready_messages.append(message)
         self.messages[:] = push_back_messages
-        self.in_flight.extend(ready_messages)
+        for m in ready_messages:
+            self.in_flight[m.message_id] = m
         return ready_messages
 
     def delete_messages(self, Entries):
@@ -157,17 +158,19 @@ def delete_messages(self, Entries):
         See: https://boto3.amazonaws.com/v1/documentation/api/latest/reference/
              services/sqs.html#SQS.Queue.delete_messages
         """
-        message_ids = {entry["Id"] for entry in Entries}
+        found_entries = []
+        not_found_entries = []
 
-        successfully_deleted = set()
-
-        for i, message in enumerate(self.in_flight):
-            if message.message_id in message_ids:
-                successfully_deleted.add(message.message_id)
-                del self.in_flight[i]
+        for e in Entries:
+            if e["Id"] in self.in_flight:
+                found_entries.append(e)
+                self.in_flight.pop(e["Id"])
+            else:
+                not_found_entries.append(e)
 
         return {
-            "Successful": [{"Id": _id} for _id in successfully_deleted],
+            "Successful": [{"Id": e["Id"]} for e in found_entries],
+            "Failed": [{"Id": e["Id"]} for e in not_found_entries],
         }
 
     def change_message_visibility_batch(self, Entries):
@@ -175,25 +178,26 @@ def change_message_visibility_batch(self, Entries):
         Changes message visibility by looking at in-flight messages, setting
         a new execute_at, and returning it to the pool of processable messages
         """
-        edited = []
-        return_to_pool = []
-        entries_by_id = {e["Id"]: e for e in Entries}
-
-        for i, m in enumerate(self.in_flight):
-            if m.message_id in entries_by_id.keys():
-                sec = int(entries_by_id[m.message_id]["VisibilityTimeout"])
+        found_entries = []
+        not_found_entries = []
+
+        for e in Entries:
+            if e["Id"] in self.in_flight:
+                found_entries.append(e)
+                in_flight_message = self.in_flight[e["Id"]]
+                sec = int(e["VisibilityTimeout"])
                 now = datetime.datetime.utcnow()
                 execute_at = now + datetime.timedelta(seconds=sec)
-                changed = attr.evolve(m, execute_at=execute_at)
-                changed.attributes["ApproximateReceiveCount"] += 1
-                edited.append(changed)
-                return_to_pool.append(changed)
-                del self.in_flight[i]
-
-        self.messages.extend(return_to_pool)
+                updated_message = attr.evolve(in_flight_message, execute_at=execute_at)
+                updated_message.attributes["ApproximateReceiveCount"] += 1
+                self.messages.append(updated_message)
+                self.in_flight.pop(e["Id"])
+            else:
+                not_found_entries.append(e)
 
         return {
-            "Successful": [{"Id": _id} for _id in edited],
+            "Successful": [{"Id": e["Id"]} for e in found_entries],
+            "Failed": [{"Id": e["Id"]} for e in not_found_entries],
         }
 
     def delete(self):