Here's a list I've complied, let me know if I've missed any:

• 6/1/2009 - Abdulhakim Mujahid Muhammad shot up a military recruiting office in Little Rock. He was investigated by the FBI 6 months prior. Link
• 11/5/2009 - Nidal Malik Hasan killed 13 in Fort Hood 5 months after being investigated by the FBI after ntercepted at least 18 e-mails between him and Anwar al-Awlaki. Link
• 12/25/2009 - Umar Abdul Mutallab attempted to blow up NWA Flight 253. British Intelligence and Umar's own father warned the CIA he was planning an attack two months prior. Link
• 4/15/2013 - Brothers Tamerlan and Dzhokhar Tsarnaev detonated two bombs near the finish line of the Boston Marathon, killing 3 spectators plus hundreds were injured, later two cops and one brother were also killed. Russian intelligence informed the FBI that they believed them to be terrorists, and the FBI investigated them.
• 4/27/2014-6/25/2014 - Ali Muhammad Brown goes on a killing spree, killing 4 civilians. He had previously served time in a federal prison for bank fraud which authorities believed as used to finance militant Islamist terrorist groups. Link
• 5/3/2015 - Two men who attacked officers with gunfire at the entrance to an exhibit featuring cartoon images of Muhammad in Garland, Texas. One of them had previously been convicted of making a false statement about terrorism, after the FBI established that he discussed traveling to Somalia to engage in "violent jihad". Link
• 12/5/2015 - Syed Rizwan Farook and Tashfeen Malik killed 14 people at a holiday party in San Bernidno. Homeland Security had investigated Tashfeen Malik just 5 month prior. Link
• 2/11/2016 - Mohammad Barry attacked an Ohio restaurant with a Jewish proprietor with a machete, and was gunned down by cops. He had been investigated by the FBI 4 years prior. Link
• 6/12/2016 - Omar Mateen killed 49 people at a gay nightclub in Orlando, FL. The FBI investigated him 2 years prior. Link
• 9/16/2016-9/19/2016 - Three bombs exploded injuring 31 and several explosive devices were found in the New York metropolitan area. The bomber, Ahmad Khan Rahimi, was reported to the FBI by his father 2 years prior. Link
• 11/28/2016 - Abdul Razak Ali Artan rammed his car into a crowd of people at Ohio State University, then got out and started stabbing. He was cleared to emmigrate to this country two years ago by the State Department and Homeland Security. Link
• 1/6/2017 - Five people were killed, six others injured in a shooting at a Fort Lauderdail airport. The shooter visited the FBI field office two months prior and said that the US government was controlling his mind and forcing him to join ISIS. Link

This can be taken one of two ways: 1) They are in on it. 2) They aren't fully capable of providing security, and the promises they make shouldn't be trusted, e.g. Russian hacking and their ability to vet refugees.