MADISON, Wis. (AP) — Wisconsin health officials told vaccinators on Tuesday to suspend use of the Johnson & Johnson shot amid fears the shots may cause blood clots.

The Centers for Disease Control and the Food and Drug Administration on Tuesday recommended vaccinators pause the shots while they investigate whether the Johnson & Johnson shots are causing unusual clots. FDA officials say the pause could last days.

The clots occurred in veins that drain blood from the brain and occurred together with low platelets, the fragments in blood that normally form clots. All six cases were in women between the ages of 18 and 48. One person died, and all of the cases remain under investigation.

More than 6.8 million doses of the J&J vaccine have been given in the U.S., the vast majority with no or mild side effects.

Wisconsin Department of Health Services Secretary Karen Timberlake said the clots appear to be extremely rare but the state pause order comes out of an abundance of caution.