After the report is made, the government agency might or might not contact the employer to investigate. In most cases, the government is under no duty to investigate. The government gets to pick and choose which issues to pursue. If they do pursue your report, the employer may begin to wonder who tipped off the government.
You might also consider that sometimes (hopefully rarely), people working in government disclose who tipped them off. Most agencies have rules against this, but some have open records laws that require disclosure.
Sometimes the employer makes a guess and starts to retaliate. If you complained anonymously, you might then have a problem proving that the employer knew or suspected that you had engaged in protected activity.