diff --git a/cartographer/mapping/connected_components.cc b/cartographer/mapping/connected_components.cc
index 22a8115..c9e8601 100644
--- a/cartographer/mapping/connected_components.cc
+++ b/cartographer/mapping/connected_components.cc
@@ -92,6 +92,7 @@ std::vector<std::vector<int>> ConnectedComponents::Components() {
 }
 
 std::vector<int> ConnectedComponents::GetComponent(const int trajectory_id) {
+  common::MutexLocker locker(&lock_);
   const int set_id = FindSet(trajectory_id);
   std::vector<int> trajectory_ids;
   for (const auto& entry : forest_) {
diff --git a/cartographer/mapping_2d/sparse_pose_graph.h b/cartographer/mapping_2d/sparse_pose_graph.h
index 4bc12ea..d0ad102 100644
--- a/cartographer/mapping_2d/sparse_pose_graph.h
+++ b/cartographer/mapping_2d/sparse_pose_graph.h
@@ -247,7 +247,8 @@ class SparsePoseGraph : public mapping::SparsePoseGraph {
     ~TrimmingHandle() override {}
 
     int num_submaps(int trajectory_id) const override;
-    void MarkSubmapAsTrimmed(const mapping::SubmapId& submap_id) override;
+    void MarkSubmapAsTrimmed(const mapping::SubmapId& submap_id)
+        REQUIRES(parent_->mutex_) override;
 
    private:
     SparsePoseGraph* const parent_;
diff --git a/cartographer/mapping_3d/sparse_pose_graph.h b/cartographer/mapping_3d/sparse_pose_graph.h
index e89fbfa..b8295e2 100644
--- a/cartographer/mapping_3d/sparse_pose_graph.h
+++ b/cartographer/mapping_3d/sparse_pose_graph.h
@@ -251,7 +251,8 @@ class SparsePoseGraph : public mapping::SparsePoseGraph {
     ~TrimmingHandle() override {}
 
     int num_submaps(int trajectory_id) const override;
-    void MarkSubmapAsTrimmed(const mapping::SubmapId& submap_id) override;
+    void MarkSubmapAsTrimmed(const mapping::SubmapId& submap_id)
+        REQUIRES(parent_->mutex_) override;
 
    private:
     SparsePoseGraph* const parent_;