Reg­u­la­tors at the SEC are sound­ing an alarm fol­low­ing sus­pi­cious trad­ing ac­tiv­i­ty that went down days be­fore Sanofi’s $11.6 bil­lion buy­out of he­mo­phil­ia drug­mak­er Biover­a­tiv.

The agency has filed a law­suit in the fed­er­al dis­trict court in Man­hat­tan against one — or pos­si­bly many — un­known traders, al­leg­ing in­sid­er knowl­edge led to mas­sive prof­its on the ac­qui­si­tion deal. The law­suit seeks an or­der that would force Cred­it Su­isse, the glob­al bank that processed the trades through an ac­count in Zurich, to dis­close the names of the trad­er(s).

In the pur­chase that raised red flags for reg­u­la­tors, traders paid about $170,000 for op­tions that earned them about $4.9 mil­lion af­ter the ac­qui­si­tion an­nounce­ment. That would be a 2,900% re­turn in less than two weeks.

When Sanofi $SNY agreed to pur­chase Biover­a­tiv $BIVV in the biggest biotech M&A deal so far in 2018, the small­er com­pa­ny’s stock price soared over 60% and its mar­ket cap climbed from $6.8 bil­lion to over $11 bil­lion.

Ap­par­ent­ly, traders want­ed to take ad­van­tage of those gains. The trans­ac­tions in ques­tion were weird on a few lev­els. First, the traders bought call op­tions that had a strike price be­tween $65 and $70 per share. That’s an un­usu­al move for a trad­er, be­cause shares of Biover­a­tiv had nev­er closed above $64.12. Why buy call op­tions at a pre­mi­um?

The oth­er strange thing about this move was the call op­tions had an ex­pi­ra­tion date of Feb­ru­ary 16, which means the buy­ers ex­pect­ed the share price to go up in a very short time. Last weird thing? The traders had nev­er owned Biover­a­tiv stock be­fore.

Oth­er than forc­ing Cred­it Su­isse to cough up the de­fen­dants’ names, the SEC is seek­ing an or­der that would freeze the traders’ as­sets. They’re al­so ask­ing that the traders hand over prof­its and pay fines.