Suspicious betting patterns have raised concerns that three matches in this year’s Wimbledon tournament may have been fixed, according to the Tennis Integrity Unit, the independent body charged with investigating corruption in the sport.

The watchdog group did not specify which matches had aroused concerns, but said that two took place during qualifying rounds, the earliest phase of the tournament, and that the third was during the main part of the competition.

Last year a BuzzFeed News and BBC investigation revealed evidence that the sport’s authorities had turned a blind eye to widespread match-fixing. Since then, the integrity unit has more than doubled its personnel. Tennis is also undergoing an independent review into "integrity in the sport."

The news about Wimbledon was released in the integrity unit’s quarterly briefing report, which mentioned a total of 53 matches, including one at the French Open. The report cautions that “an alert on its own is not evidence of match-fixing,” but may prompt further inquiry.

Still, the three alerts from tennis’s most famous tournament represent a striking development. Most alerts, which are passed along to the sport’s authorities by major betting operators, involve play at lower levels of competition. Alerts about matches in tennis’s top tiers are rare; this is only the second time one has been announced about Wimbledon.

The integrity unit received 83 alerts during the first half of 2017, down from 121 during the same period in 2016.