diff --git a/ortools/util/fp_utils.cc b/ortools/util/fp_utils.cc
index 734b2f031f..f40a81dad8 100644
--- a/ortools/util/fp_utils.cc
+++ b/ortools/util/fp_utils.cc
@@ -22,7 +22,6 @@
 #include <cstdlib>
 #include <limits>
 #include <utility>
-#include <vector>
 
 #include "absl/base/casts.h"
 #include "absl/log/check.h"
diff --git a/ortools/util/fp_utils.h b/ortools/util/fp_utils.h
index ad2718b1dd..4590ce1167 100644
--- a/ortools/util/fp_utils.h
+++ b/ortools/util/fp_utils.h
@@ -29,7 +29,6 @@
 #include <limits>
 // Needed before fenv_access. See https://github.com/microsoft/STL/issues/2613.
 #include <numeric>  // IWYU pragma:keep.
-#include <vector>
 
 #include "absl/log/check.h"
 #include "absl/types/span.h"
diff --git a/ortools/util/sigint.cc b/ortools/util/sigint.cc
index b807372d65..601f4983cc 100644
--- a/ortools/util/sigint.cc
+++ b/ortools/util/sigint.cc
@@ -23,15 +23,18 @@ namespace operations_research {
 
 void SigintHandler::Register(const std::function<void()>& f) {
   handler_ = [this, f]() -> void {
-    ++num_sigint_calls_;
-    if (num_sigint_calls_ >= 3) {
-      LOG(INFO) << "^C pressed " << num_sigint_calls_
-                << " times. Forcing termination.";
+    const int num_sigint_calls = ++num_sigint_calls_;
+    if (num_sigint_calls < 3) {
+      LOG(INFO)
+          << "^C pressed " << num_sigint_calls << " times. "
+          << "Interrupting the solver. Press 3 times to force termination.";
+      if (num_sigint_calls == 1) f();
+    } else if (num_sigint_calls == 3) {
+      LOG(INFO) << "^C pressed 3 times. Forcing termination.";
       exit(EXIT_FAILURE);
+    } else {
+      // Another thread is already running exit(), do nothing.
     }
-    LOG(INFO) << "^C pressed " << num_sigint_calls_ << " times. "
-              << "Interrupting the solver. Press 3 times to force termination.";
-    if (num_sigint_calls_ == 1) f();
   };
   signal(SIGINT, &ControlCHandler);
 }
diff --git a/ortools/util/sigint.h b/ortools/util/sigint.h
index 457de9bd67..7b3098033e 100644
--- a/ortools/util/sigint.h
+++ b/ortools/util/sigint.h
@@ -14,6 +14,7 @@
 #ifndef OR_TOOLS_UTIL_SIGINT_H_
 #define OR_TOOLS_UTIL_SIGINT_H_
 
+#include <atomic>
 #include <functional>
 
 namespace operations_research {
@@ -30,7 +31,7 @@ class SigintHandler {
  private:
   static void ControlCHandler(int s);
 
-  int num_sigint_calls_ = 0;
+  std::atomic<int> num_sigint_calls_ = 0;
   thread_local static std::function<void()> handler_;
 };