Sens. Richard Blumenthal and Marsha Blackburn said they would launch a probe into Facebook Inc.’s internal research on the way its Instagram photo- and video-sharing service affects young users, prompted by a Wall Street Journal investigation that showed the company knew the app was harmful to some in that group.

The Democratic chairman and ranking Republican on the Consumer Protection, Product Safety, and Data Security Subcommittee on Tuesday also said that they were in touch with someone they identified as “a Facebook whistleblower”...

Sens. Richard Blumenthal and Marsha Blackburn said they would launch a probe into Facebook Inc.’s internal research on the way its Instagram photo- and video-sharing service affects young users, prompted by a Wall Street Journal investigation that showed the company knew the app was harmful to some in that group.

The Democratic chairman and ranking Republican on the Consumer Protection, Product Safety, and Data Security Subcommittee on Tuesday also said that they were in touch with someone they identified as “a Facebook whistleblower” and “will use every resource at our disposal to investigate what Facebook knew and when they knew it—including seeking further documents and pursuing witness testimony.” In the statement, the senators said, “The Wall Street Journal’s blockbuster reporting may only be the tip of the iceberg.”

Facebook’s research, which was conducted over the past three years, found that the app was especially damaging to some teenage girls. According to one slide from 2019, seen by the Journal, researchers concluded, “we make body image issues worse for one in three teen girls.” Teens surveyed by the company also blamed Instagram for increases in anxiety and depression, according to the research.

Facebook executives have frequently played down Instagram’s negative effects in public and withheld internal findings about their research. In March, Facebook Chief Executive Mark Zuckerberg said the company was “early in thinking through” a version of Instagram tailored for children under 13.

“The WSJ’s story today on research we’re doing to understand young people’s experiences on [Instagram] casts our findings in a negative light, but speaks to important issues,” Instagram CEO Adam Mosseri said in a tweet Tuesday. “We stand by this work and believe more companies should be doing the same.”

The company also issued a blog post saying its internal research tracked with external findings that showed the effects of social media on people’s well-being is “mixed.” Facebook declined to comment directly on the statements from lawmakers.

In April, Democratic lawmakers sent Mr. Zuckerberg a letter criticizing his company’s effort to launch an Instagram for children and, in May, attorneys general from 44 states and territories urged Facebook to abandon the plans.

In August, Sens. Blumenthal from Connecticut and Blackburn from Tennessee—who have both criticized the company in the past—called on Mr. Zuckerberg to release Facebook’s internal research on the impact of its platforms on youth mental health. The company responded with a six-page letter that didn’t include its own findings.

“It is clear that Facebook is incapable of holding itself accountable,” Sens. Blumenthal and Blackburn said in the joint statement. “The Wall Street Journal’s reporting reveals Facebook’s leadership to be focused on a growth-at-all-costs mindset that valued profits over the health and lives of children and teens.”

“When given the opportunity to come clean to us about their knowledge of Instagram’s impact on young users, Facebook provided evasive answers that were misleading and covered up clear evidence of significant harm,” the senators added.

The consumer protection subcommittee is part of the Senate Commerce Committee.

The Journal’s investigation into Facebook is based on an extensive cache of internal documents. At least some of those documents have been turned over to the Securities and Exchange Commission and to Congress by a person seeking federal whistleblower protection, the Journal previously reported.

“It’s clear from these documents that Facebook made deliberate decisions that enabled the spread of misinformation and other harmful content. Lawful, anonymous disclosures like the ones here are a critical way for law enforcement and Congress to do their work,” said John Tye, a lawyer with Whistleblower Aid, who filed the protected disclosures with the SEC.

Facebook didn’t immediately respond to a request for comment on the lawyer’s statement.

Lawmakers on both sides of the Atlantic expressed alarm Tuesday after the Journal’s reporting on Instagram’s internal research.

“This is appalling,” Sen. Ed Markey (D., Mass.) wrote on Twitter. “I’ll be demanding answers from Mark Zuckerberg.”

Damian Collins, a British member of Parliament who is spearheading an online safety bill, also blasted the company saying in a statement that the Journal’s findings “exposed how the company, time and again, puts profit before harm.”

He added: “Its own research is telling it that a large number of teen Instagram users say the service makes them feel worse about themselves—but the company just wants to make sure they keep coming back.”

Write to Deepa Seetharaman at Deepa.Seetharaman@wsj.com