Inclusion Criteria:  Diagnosis of Crohn Disease (CD) for at least 3 months prior to visit 1 by endoscopic, radiology, and supported by histology. Elevated C-reactive protein (≥ 5 mg/L) OR elevated fecal calprotectin (≥ 250 µg/g) Moderate to severe active CD at visit 1 defined as Crohn's Disease Activity Index (CDAI) ≥220 and ≤450. Presence of mucosal ulcers in at least one segment of the ileum or colon and a Simple Endoscopic Score for Crohn's disease (SES-CD) score ≥ 7 (for patients with isolated ileitis ≥4). Patients who are experienced to 1 or 2 tumor necrosis factor (TNF) antagonists at a dose approved for CD. Patients may have stopped TNF antagonists treatment due to primary or secondary non-responsiveness, intolerance, or for other reasons.  May be receiving a therapeutic dose of the following:  Oral 5-aminosalicylic acid (5-ASA) compounds must have been at a stable dose for at least 4 weeks prior to randomisation and must continue on this dose until week 12 and/or Oral corticosteroids if indicated for treatment of CD must be at a prednisone equivalent dose of ≤ 20 mg/day, or ≤ 9 mg/day of budesonide, and have been at a stable dose for at least 2 weeks immediately prior to randomisation and must continue on this dose until week 12. and/or Azathioprine (AZA), mercaptopurine (MP), or methotrexate (MTX), provided that dose has been stable for the 8 weeks immediately prior to randomisation and must continue on this dose until week 12. Women of childbearing potential must be ready and able to use highly effective methods of birth control. Further inclusion criteria apply  Exclusion Criteria:  Have any current or prior abscesses, unless they have been drained and treated at least 6 weeks prior to randomisation and are not anticipated to require surgery. Patients with active fistulas may be included if there is no anticipation of a need for surgery and there are currently no abscesses present based on investigator's judgement. Have complications of CD such as strictures, stenosis, short bowel syndrome, or any other manifestation that might require surgery, or could preclude the use of SES-CD/CDAI to assess response to therapy, or would possibly confound the evaluation of benefit from treatment with BI 706321 (based on investigator's judgement). Patient with an inflammatory bowel disease (IBD) diagnosis other than CD. Have had any kind of bowel resection or diversion within 4 months or any other intra-abdominal surgery within 3 months prior to visit 1. Patients with current ileostomy, colostomy, or ileorectal anastomosis are excluded.  Treatment with:  Any non-biologic medication for IBD (e.g.tacrolimus or mycophenolate mofetil, systemic corticosteroids), other than those allowed per inclusion criteria, within 30 days prior to randomisation Any biologic treatment with a TNF-alpha antagonist (adalimumab, infliximab, golimumab, certolizumab pegol) or vedolizumab within 4 weeks prior to randomisation. Any previous treatment with ustekinumab Any previous treatment with an investigational non[1]biologic or biologic drug for CD (including but not limited to JAK inhibitors, S1P modulators, interleukin (IL)-23 inhibitors, anti-integrins). Any investigational drug for an indication other than CD during the course of the actual study and within 30 days or 5 half-lives (whichever is longer) prior to randomisation. Any prior exposure to rituximab within 1 year prior to randomisation. Positive stool examination for C difficile or other intestinal pathogens <30 days prior to randomization Evidence of colonic moderate/severe mucosal dysplasia or colonic adenomas, unless properly removed Increased risk of infectious complications (e.g. recent pyogenic infection, any congenital or acquired immunodeficiency (e.g. human immunodeficiency virus (HIV)), past organ or stem cell transplantation (with exception of a corneal transplant > 12 weeks prior to screening) or have ever received stem cell therapy (e.g., Prochymal). Prior treatment with a somatic cell therapy product (e.g., Alofisel) is not excluded, provided it was administered > 8 weeks prior to randomisation. Live or attenuated vaccination within 4 weeks prior to randomisation. Presence of clinically significant acute or chronic infections not otherwise listed, including viral hepatitis, COVID-19, or others based on investigator's judgement. A marked baseline prolongation of QT/QTc interval (such as QTcF intervals that are greater than 450 ms for men, 470 ms for female) or any other relevant electrocardiogram (ECG) finding at screening. Both have to be confirmed by repeated ECG recording. Further exclusion criteria apply