In what circumstances will a prosecution gang expert testify to establish the elements of the crime of gang participation under section 186.22, subdivision (a)?

In People v. Thomas, a prosecution gang expert testified to establish the elements of the offense of gang participation under section 186.22, subdivision (a). (People v. Thomas, supra, 130 Cal.App.4th at pp. 1205, 1207.) The expert testified much of his expertise came from statements made by other officers and by gang members. (Id. at p. 1207.) His opinion that the defendant was a gang member was based in part on information he found in police reports and statements of gang members who said the defendant was a member. (Id. at p. 1206.) The defendant argued the admission of the gang expert's testimony about the statements of other gang members violated the confrontation clause as interpreted in Crawford v. Washington. (People v. Thomas, supra, at p. 1208.)