With the surge in online sales during the holidays came a wave of attempted criminal activity, and the 2016 holiday shopping season experienced a 31% year-over-year increase in global fraudulent purchase attempts, according to a new study. The rate of fraudulent transaction attempts spiked on Christmas Eve and express shipping cutoff days, which are not universal across e-retailers but often fall around Dec. 20 and Dec. 21.

ACI Worldwide’s electronic payments services are used by more than 5,100 businesses, including financial institutions and merchants, and its report is based on an analysis of hundreds of millions of transactions from e-retailers worldwide. ACI did not disclose the countries from which purchase data was included in the study; a spokeswoman says the analysis was weighted heavily in the U.S., although ACI operates in nearly every country. The data was collected from Thanksgiving Day through Dec. 31 in 2016 and 2015. ACI sells fraud-detection and -prevention service used for transactions initiated without the cardholder and card being physically present. The fraud-screening platform processed 7% of online spending on Black Friday (the day after Thanksgiving) and 13% of web spend on Cyber Monday (the Monday after Thanksgiving), according to ACI.

The fraud-attempt rate was calculated by classifying several scenarios, including when transaction data was confirmed as fraudulent by a merchant, when the transaction data matched a record in ACI’s database that tracks reports of chargebacks (where a payment card provider requires a merchant to refund a criminal transaction), credit card numbers sold online in underground criminal forums or reports of fraud by an issuer.

While ACI did not compare the holiday season to general annual fraud rates, a spokeswoman says as e-retail continues to proliferate, fraud numbers grow, and 2016 generated the most online shopping fraud ACI has tracked in late November and December.

“Over the 2016 holiday shopping season, merchants experienced significant growth in their digital channels, coupled with a substantial increase in fraud,” says Markus Rinderer, senior vice president of Platform Solutions at ACI Worldwide. “Given the consistent and alarming uptick in fraudulent activity on key dates, merchants must be proactive in their efforts to identify weak points—and define short- and long-term strategies for improved security and enhanced customer experience.”

Other key findings:

• The number of online retail transactions jumped 16% during the 2016 holiday season compared with the same period in 2015. ACI did not release the number of transactions.
• One of every 97 online transactions was deemed a fraudulent attempt during the 2016 holiday season, up from one of every 109 in 2015.
• Of all transactions made on Christmas Eve, 1.6% were fraudulent. On the standard shipping shipment cutoff date, which varies by merchant, 1.5% were counterfeit. And for the express shipment cutoff date, which also depends on the e-retailer, 1.7% were flagged as fake.
• The average ticket price of an attempted fraudulent transaction during the 2016 holiday season was $227.84, down 6.1%  from $242.58 in 2015. This trend was driven by reduced shipping fees, lower-priced electronics and promotions.
• Transaction volume on Black Friday increased 19% year over year, and on Cyber Monday increased 15%.