diff --git a/lib/models/user.js b/lib/models/user.js
index 4ad2fde7..7a062da7 100644
--- a/lib/models/user.js
+++ b/lib/models/user.js
@@ -18,6 +18,7 @@ var Model = require('../loopback').Model
   , ACL = require('./acl').ACL
   , assert = require('assert');
 
+var debug = require('debug')('loopback:user');
 /**
  * Default User properties.
  */
@@ -143,20 +144,24 @@ User.login = function (credentials, fn) {
     var defaultError = new Error('login failed');
     
     if(err) {
+      debug('An error is reported from User.findOne: %j', err);
       fn(defaultError);
     } else if(user) {
       user.hasPassword(credentials.password, function(err, isMatch) {
         if(err) {
+          debug('An error is reported from User.hasPassword: %j', err);
           fn(defaultError);
         } else if(isMatch) {
           user.accessTokens.create({
             ttl: Math.min(credentials.ttl || User.settings.ttl, User.settings.maxTTL)
           }, fn);
         } else {
+          debug('The password is invalid for user %s', query.email || query.username);
           fn(defaultError);
         }
       });
     } else {
+      debug('No matching record is found for user %s', query.email || query.username);
       fn(defaultError);
     }
   });