(Bloomberg) -- Bitcoin retreated Monday back toward $40,000 after climbing over the weekend to the highest levels since May.

The largest cryptocurrency fell as much as 3% and was trading at $40,380 as of 7:03 a.m. in Hong Kong. Other virtual coins including second-ranked Ether also fell. A trigger for the declines beyond possible profit-taking wasn’t immediately apparent.

Bitcoin has rallied over the past two weeks, breaking past the upper bound of a prolonged trading range between $30,000 and $40,000. It’s been bolstered by supportive comments from billionaire Elon Musk and Ark Investment Management LLC’s Cathie Wood, as well as speculation over Amazon.com Inc.’s possible involvement in the cryptocurrency sector.

At the same time, scrutiny of the industry is intensifying. That includes a push by U.S. legislators for stricter rules on cryptocurrency investors to collect more taxes to fund a portion of a planned $550 billion investment into transportation and power systems.

©2021 Bloomberg L.P.