diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index c816668..9d7b72f 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -1072,6 +1072,10 @@
     }
 
 #if defined(MBEDTLS_THREADING_C)
+    /* We cannot unlock between setting the state to PENDING_DELETION
+     * and destroying the key in storage, as otherwise another thread
+     * could load the key into a new slot and the key will not be
+     * fully destroyed. */
     PSA_THREADING_CHK_GOTO_EXIT(mbedtls_mutex_lock(
                                     &mbedtls_threading_key_slot_mutex));
 #endif