The stock market continued its slump Friday, as investors appeared to lose confidence in formerly high-flying stocks that combine high-growth potential with high risk. With many pointing to valuations as having climbed too far, momentum stocks were among the most-punished investments on the market today. Leading the way down were Herbalife (NYSE:HLF), voxeljet (NYSE:VJET), and J.C. Penney (OTC:JCPN.Q), each of which fell substantially more than the 1% to 1.5% losses for the broader market.

Source: Herbalife.

Herbalife dropped 14%, with the decline coming late in the day after reports that the seller of nutritional and personal-care products was the subject of a probe from the FBI and the Department of Justice. Herbalife said, after the market closed, that it had no knowledge of such an investigation and that it has not yet received formal notification from either of the federal agencies. But with two separate news sources reporting at least the FBI investigation, investors sold first and asked questions later. The real question that long-term investors should ask, though, is whether they think a probe will actually result in any substantial punishment for Herbalife, which has become the battleground for fights among prominent hedge-fund managers on opposite sides of trading positions in Herbalife stock. The rub here, though, is that a potential criminal investigation ups the ante from an earlier-announced probe from the Federal Trade Commission.

The 19% plunge in voxeljet shares today came after the pricing of its 3 million share secondary offering came in far lower than existing shareholders wanted to see. After having traded as high as $70 per share last November, voxeljet was willing to accept just $15 per share in its offering, signaling the level of desperation that the 3-D printer has to get working capital. The capital-raising fire sale certainly can't give investors much confidence in voxeljet's own assessment of its future prospects, as otherwise, voxeljet would have postponed the offering and protected investors from the huge dilution that will now result from the stock sale.

J.C. Penney fell almost 10% as one of the real estate investment trusts that owns malls where the ailing retailer is an anchor tenant said that it expects to see J.C. Penney close more of its stores in the future. J.C. Penney has a strong opportunity during 2014 to rebuild on what should be remarkably easy comparisons to a horrible 2013, which could help restore confidence, at least among those investors who focus solely on year-over-year comps. In the longer run, though, J.C. Penney needs to address the difficult issues that initially led it to try its failed experiment to shift from its traditional discounting business model. Unless it can both recapture its lost customer base and woo new shoppers into its stores, J.C. Penney's future doesn't look bright.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.