Proceeds from initial coin offerings have topped $4 billion this year despite escalating warnings from U.S. officials of rising risks and fraud.

About a quarter of that investment went into startups developing core technology, such as new kinds of distributed ledgers, according data from Autonomous Research LLP. Companies focused on revolutionizing finance and on letting machines exchange information garnered large shares of the total as well.

U.S. Securities and Exchange Commission Chairman Jay Clayton reiterated Monday that many token sales are effectively securities that should be registered with the agency. Clayton added that investors should be on high alert for promoters promising guaranteed returns, and he warned market participants about touting ICOs without determining whether they comply with securities regulations.