The Pennsylvania Department of Revenue flagged more than 31,000 personal income tax returns last year for potential identity-theft fraud, up from 8,521 in 2015 and 1,132 in 2014, the department said Wednesday.

Since the department increased its vigilance by forming a Personal Income Tax Fraud Investigation Unit early last year, it blocked $6.4 million in fraudulent returns involving identity theft through December, it said.

When the automated fraud-detection system sees a potential problem and information can't be verified, the department, which received 6.4 million returns last year, sends a letter to taxpayers telling them how they can verify their identity.

The IRS last week renewed its warning about a spear-phishing email scam that uses a corporate officer's name to request employee Forms W-2 from company payroll or human resources departments. When it works, as it did last year at Main Line Health, the cybercriminals use the information to, at the very least, file fraudulant tax returns.