diff --git a/axon/services/citations.py b/axon/services/citations.py
new file mode 100644
index 0000000..05c94e8
--- /dev/null
+++ b/axon/services/citations.py
@@ -0,0 +1,11 @@
+import re
+
+from axon.models import SearchResult
+
+_CITATION_RE = re.compile(r"\[(\d+)\]")
+
+
+def validate_citations(report: str, sources: list[SearchResult]) -> tuple[int, list[int], bool]:
+    citations = sorted({int(match) for match in _CITATION_RE.findall(report)})
+    missing = [citation for citation in citations if citation < 1 or citation > len(sources)]
+    return len(citations), missing, not missing and bool(citations)