Estou usando o AFNetworking para algumas consultas GET. Mas minha função sempre retorna valor nulo. Onde eu estava errado?

+ (NSString *)getRequestFromUrl:(NSString *)requestUrl {
    NSString * completeRequestUrl = [NSString stringWithFormat:@"%@%@", BASE_URL, requestUrl];
    __block NSString * results;
    AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
    [manager GET:completeRequestUrl parameters:nil success:^(AFHTTPRequestOperation *operation, id responseObject) {
        results = [NSString stringWithFormat:@"%@", responseObject];
    } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
        results = [NSString stringWithFormat:@"Error"];
    }];
    NSLog(@"%@", results);
    return results;
}

THX! Artem.