WASHINGTON (AP) — The U.S. is recommending a “pause” in using the single-dose Johnson & Johnson COVID-19 vaccine to investigate reports of potentially dangerous blood clots.

The Centers for Disease Control and Prevention and the Food and Drug Administration said Tuesday they were investigating unusual clots that occurred 6 to 13 days after vaccination. The clots occurred in veins that drain blood from the brain and occurred together with low platelets. All six cases were in women between the ages of 18 and 48; there was one death and all remained under investigation.