AECOM Environmental Engineering interview questions

The whole process is smooth. I applied online; recruiter emailed me to arrange phone screen; recruiter sent my profile and answers to the hiring manager; the hiring managers (the project managers of the project I am doing) say they want to interview me, then arranged an interview just after the second day of the phone screen. But I waited more than 3 weeks to get the offer. I tried to email them or inmail then on Linkedin within the 3 weeks but no response. Suddenly after 3 weeks, I got the offer. Getting the offer is kinda long too because they screened and checked my background information twice, one unofficially before they actually submit it to the system of the company, and one officially.