Who called the fire alarm about Montag?

At the beginning of Part 3, Montag joins Captain Beatty on a routine fire call and is astonished to arrive at his home, where Captain Beatty tells Montag that he must destroy his own home and book collection. As Montag steps out of the Salamander, Mildred runs out of their home carrying her suitcases and muttering about the parlor wall 'Family.' Montag immediately questions Mildred and asks if she called in the alarm. Mildred does not respond to Montag's question, refuses to look him in the eye, and gets into a taxi. Captain Beatty then forces Montag to set his home and book collection on fire using a flamethrower.
After Montag is finished burning his home and belongings, Captain Beatty attempts to place him under arrest. When Montag asks Beatty if Mildred called in the alarm, he confirms that she did by nodding yes and saying,

"But her friends turned in an alarm earlier, that I let ride. One way or the other, you'd have got it. It was pretty silly, quoting poetry around free and easy like that" (Bradbury, 54).

Overall, Mildred and her friends called in separate alarms on Montag.