The 2401 attack on the Starfleet Recruitment Center in San Francisco using a portal weapon stolen from Daystrom Station.
Early in 2401 a portal weapon was stolen from Daystrom Station, Starfleet's most secure research facility. The weapon was then used to strike the Starfleet Recruitment Center in San Francisco, killing and injuring people there.
Raffi Musiker, working undercover for Starfleet Intelligence with Worf as her handler, investigated the theft. The trail led through the criminal underworld of M'Talas Prime and eventually showed that Changelings were behind it, giving one of the first signs of how widely they had infiltrated Starfleet.