Shortly after the Biden administration announced that it was taking steps toward regulating cryptocurrencies, the Department of Labor issued a message of its own: Crypto doesn’t belong in most employees’ retirement accounts — and the department intends to investigate some plans that already offer the option.

The department did not go as far as banning crypto-related investments from workplace retirement plans, which held $6.2 trillion on behalf of 91 million 401(k) participants in 2019. But the department, which oversees workplace retirement offerings, said it had grave concerns about including these emerging digital options on 401(k) investment menus, according to department officials and a compliance assistance document released on Thursday.

The department also said it expected to conduct an investigative program aimed at plans offering crypto and related investments and would “take appropriate action” to protect employee participants.

“These assets are for people’s old age, and we really are very serious about making sure that this money is invested and recommendations are made in people’s best interest,” Ali Khawar, acting assistant secretary at the department’s Employee Benefits Security Administration, said in an interview.

Retirement plan administrators — who must act solely in the best interest of the employees participating — are responsible for choosing prudent investment options. If they include what could be deemed an imprudent option, and leave it to the worker to decide its merits, that would amount to a failure of fiduciary duty, the department said.

Cryptocurrencies aren’t widely used in retirement plans, Mr. Khawar said. But the department began developing guidance after it heard that more 401(k) plans were receiving pitches.

“Cryptocurrencies are very different from typical retirement plan investments,” the Labor Department said in the compliance document, “and it can be extraordinarily difficult, even for expert investors, to evaluate these assets and separate the facts from the hype.”