Detectives investigating alleged lockdown-busting parties in Downing Street and the Cabinet Office have begun interviewing key witnesses.
The Metropolitan Police said more than 100 questionnaires have been sent to people at the alleged meetings so far, which must be completed and returned within seven days.
A No 10 source said on Monday evening that Prime Minister Boris Johnson has not been interviewed.
Investigators have begun reviewing all available evidence but have not yet made referrals for potential fines.
The Met said in a statement: “In addition to the detailed review of all available material, including returned questionnaires, detectives from the Operation Hillman investigation team have begun interviewing key witnesses.
“This research includes a significant amount of research material; providing over 100 questionnaires and the need to rate each response individually.
“The violations in question involve a number of elements and the legislation itself has changed between the dates of the event. We are progressing the investigation as soon as possible
“Until now, we have made no references to the ACRO Criminal Record for issuing fixed fines.
“However, each response to the questionnaire will be assessed alongside all available evidence, and if it reaches the evidence threshold, referrals will occur.”
Police are investigating 12 events, including as many as six that Mr Johnson is said to have attended.
About a month ago, more than 50 questionnaires were sent out, a figure that is now outdated.
The Met said: “As a result of the responses to date, additional individuals have been identified and questionnaires have been sent to them. As the investigation progresses, we may need to reach out to more people as more information comes to light.”
Johnson has faced calls to resign due to his attendance at some of the events under investigation.