The U.S. Securities and Exchange Commission is investigating Illumina over its controversial $7.1 billion acquisition of cancer test developer Grail, the DNA sequencing company said in a securities filing late Thursday. 

Last month, the SEC informed Illumina about the probe and requested documents and communications related to the deal. The agency also asked for statements and disclosures about the “conduct and compensation” of certain members of both Illumina and Grail’s management, according to the filing. 

Illumina, in the filing, said it is cooperating with the SEC. An agency spokesperson did not immediately respond to CNBC’s request for comment on the investigation. 

Shares of Illumina fell about 4% Friday. 

The SEC’s probe only puts more pressure on Illumina, which has lost great sums of money since closing the deal in August 2021. The company’s market value has fallen to roughly $28 billion from about $75 billion the month the deal closed. 

Illumina’s Grail deal has also faced heavy scrutiny from antitrust regulators in the U.S. and European Union. 

The European Commission, the EU’s executive body, fined Illumina a record $476 million last month for closing the acquisition without first securing regulatory approval. 

The fine came after the commission blocked the deal in September over concerns it would stifle innovation and consumer choice in the emerging market for cancer detection tests. 

Illumina has appealed the European Commission’s decision, arguing that the body lacks jurisdiction to block the merger between the two U.S. companies. 

Illumina expects a final decision on an appeal in late 2023 or early 2024. That’s also when the company anticipates it will hear an outcome of its appeal of a similar order by the U.S. Federal Trade Commission. 

Illumina has said it will divest Grail if it loses either appeal. 

Illumina’s determination to keep Grail sparked a heated proxy showdown with activist investor Carl Icahn, who holds a 1.4% stake in the company. Much of Icahn’s opposition stemmed from Illumina’s decision to close the acquisition without gaining approval from antitrust regulators.

Illumina believes it can expand the availability, affordability and profitability of Grail’s Galleri test, which can screen for more than 50 types of cancers through a single blood draw.