diff --git a/include/openthread.h b/include/openthread.h
index 3edd2a190..d6935cfd1 100644
--- a/include/openthread.h
+++ b/include/openthread.h
@@ -258,11 +258,12 @@ typedef void (*otHandleActiveScanResult)(otActiveScanResult *aResult);
 ThreadError otActiveScan(uint32_t aScanChannels, uint16_t aScanDuration, otHandleActiveScanResult aCallback);
 
 /**
- * This function determines if an IEEE 802.15.4 Active Scan is currently in progress.
+ * This function indicates whether or not an IEEE 802.15.4 Active Scan is currently in progress.
+ *
+ * @returns true if an IEEE 802.15.4 Active Scan is in progress, false otherwise.
  *
- * @returns true if an active scan is in progress.
  */
-bool otActiveScanInProgress(void);
+bool otIsActiveScanInProgress(void);
 
 /**
  * This function starts a Thread Discovery scan.
@@ -280,11 +281,12 @@ ThreadError otDiscover(uint32_t aScanChannels, uint16_t aScanDuration, uint16_t
                        otHandleActiveScanResult aCallback);
 
 /**
- * This function determines if an MLE Thread Discovery is currently in progress.
+ * This function indicates whether or not an MLE Thread Discovery is currently in progress.
+ *
+ * @returns true if an MLE Thread Discovery is in progress, false otherwise.
  *
- * @returns true if an active scan is in progress.
  */
-bool otDiscoverInProgress(void);
+bool otIsDiscoverInProgress(void);
 
 /**
  * @}
diff --git a/src/core/openthread.cpp b/src/core/openthread.cpp
index 3d707313e..1c620ba8f 100644
--- a/src/core/openthread.cpp
+++ b/src/core/openthread.cpp
@@ -935,7 +935,7 @@ ThreadError otActiveScan(uint32_t aScanChannels, uint16_t aScanDuration, otHandl
                                              reinterpret_cast<void *>(aCallback));
 }
 
-bool otActiveScanInProgress(void)
+bool otIsActiveScanInProgress(void)
 {
     return sThreadNetif->GetMac().IsActiveScanInProgress();
 }
@@ -990,6 +990,11 @@ ThreadError otDiscover(uint32_t aScanChannels, uint16_t aScanDuration, uint16_t
                                            reinterpret_cast<void *>(aCallback));
 }
 
+bool otIsDiscoverInProgress(void)
+{
+    return sThreadNetif->GetMle().IsDiscoverInProgress();
+}
+
 void HandleMleDiscover(otActiveScanResult *aResult, void *aContext)
 {
     otHandleActiveScanResult handler = reinterpret_cast<otHandleActiveScanResult>(aContext);
diff --git a/src/core/thread/mesh_forwarder.cpp b/src/core/thread/mesh_forwarder.cpp
index 17cda393e..5f90b3235 100644
--- a/src/core/thread/mesh_forwarder.cpp
+++ b/src/core/thread/mesh_forwarder.cpp
@@ -103,6 +103,13 @@ ThreadError MeshForwarder::Stop()
     mPollTimer.Stop();
     mReassemblyTimer.Stop();
 
+    if (mScanning)
+    {
+        mMac.SetChannel(mRestoreChannel);
+        mScanning = false;
+        mMle.HandleDiscoverComplete();
+    }
+
     while ((message = mSendQueue.GetHead()) != NULL)
     {
         mSendQueue.Dequeue(*message);
diff --git a/src/core/thread/mle.cpp b/src/core/thread/mle.cpp
index 64bb11fbb..6719aae0f 100644
--- a/src/core/thread/mle.cpp
+++ b/src/core/thread/mle.cpp
@@ -148,6 +148,8 @@ Mle::Mle(ThreadNetif &aThreadNetif) :
     mAssignLinkQuality = 0;
     mAssignLinkMargin = 0;
     memset(&mAddr64, 0, sizeof(mAddr64));
+
+    mIsDiscoverInProgress = false;
 }
 
 ThreadError Mle::Enable(void)
@@ -218,12 +220,14 @@ ThreadError Mle::Discover(uint32_t aScanChannels, uint16_t aScanDuration, uint16
                           DiscoverHandler aCallback, void *aContext)
 {
     ThreadError error = kThreadError_None;
-    Message *message;
+    Message *message = NULL;
     Ip6::Address destination;
     Tlv tlv;
     MeshCoP::DiscoveryRequestTlv discoveryRequest;
     uint16_t startOffset;
 
+    VerifyOrExit(!mIsDiscoverInProgress, error = kThreadError_Busy);
+
     mDiscoverHandler = aCallback;
     mDiscoverContext = aContext;
     mMesh.SetDiscoverParameters(aScanChannels, aScanDuration);
@@ -253,6 +257,8 @@ ThreadError Mle::Discover(uint32_t aScanChannels, uint16_t aScanDuration, uint16
     destination.mFields.m16[7] = HostSwap16(0x0002);
     SuccessOrExit(error = SendMessage(*message, destination));
 
+    mIsDiscoverInProgress = true;
+
     otLogInfoMle("Sent discovery request\n");
 
 exit:
@@ -265,8 +271,14 @@ exit:
     return error;
 }
 
+bool Mle::IsDiscoverInProgress(void)
+{
+    return mIsDiscoverInProgress;
+}
+
 void Mle::HandleDiscoverComplete(void)
 {
+    mIsDiscoverInProgress = false;
     mDiscoverHandler(NULL, mDiscoverContext);
 }
 
@@ -2232,6 +2244,8 @@ ThreadError Mle::HandleDiscoveryResponse(const Message &aMessage, const Ip6::Mes
 
     otLogInfoMle("Handle discovery response\n");
 
+    VerifyOrExit(mIsDiscoverInProgress, error = kThreadError_Drop);
+
     offset = aMessage.GetOffset();
     end = aMessage.GetLength();
 
diff --git a/src/core/thread/mle.hpp b/src/core/thread/mle.hpp
index 1c77d541d..eb0994003 100644
--- a/src/core/thread/mle.hpp
+++ b/src/core/thread/mle.hpp
@@ -420,6 +420,14 @@ public:
     ThreadError Discover(uint32_t aScanChannels, uint16_t aScanDuration, uint16_t aPanId,
                          DiscoverHandler aCallback, void *aContext);
 
+    /**
+     * This method indicates whether or not an MLE Thread Discovery is currently in progress.
+     *
+     * @returns true if an MLE Thread Discovery is in progress, false otherwise.
+     *
+     */
+    bool IsDiscoverInProgress(void);
+
     /**
      * This method is called by the MeshForwarder to indicate that discovery is complete.
      *
@@ -1146,6 +1154,7 @@ private:
 
     DiscoverHandler mDiscoverHandler;
     void *mDiscoverContext;
+    bool mIsDiscoverInProgress;
 
     Ip6::NetifUnicastAddress mLinkLocal16;
     Ip6::NetifUnicastAddress mLinkLocal64;
diff --git a/src/ncp/ncp_base.cpp b/src/ncp/ncp_base.cpp
index a104ab2e3..b0427a5e7 100644
--- a/src/ncp/ncp_base.cpp
+++ b/src/ncp/ncp_base.cpp
@@ -1317,7 +1317,7 @@ ThreadError NcpBase::GetPropertyHandler_MAC_SCAN_STATE(uint8_t header, spinel_pr
 {
     ThreadError errorCode = kThreadError_None;
 
-    if (otActiveScanInProgress())
+    if (otIsActiveScanInProgress())
     {
         errorCode = SendPropertyUpdate(
                         header,