Questions are swirling around the police response to Tuesday's mass shooting at Robb Elementary School in Uvalde, Texas — and the timeline of the incident is coming under close scrutiny.

While police, and witnesses, initially said that officers exchanged fire with the shooter before he entered the building, a Wednesday afternoon press conference disputed that claim, with authorities saying that they never engaged with him before he killed 19 children and two adults.

Rep. Joaquin Castro, a Texas congressman who represents a neighboring district, wrote a letter on Thursday to the FBI Director Christopher Wray calling on the agency "to use its maximum authority to thoroughly examine the timeline of events and the law enforcement response and to produce a full, timely, and transparent report of your findings."

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Castro wrote that state officials "have provided conflicting accounts that are at odds with those provided by witnesses." He added that "a block of time between 11:30 a.m. and 1 p.m. … has yet to be fully accounted for. Onlookers allege that parents unsuccessfully urged law enforcement to enter the building during this time and confront the shooter."

While the motive of the attack is still unknown, certain details have emerged about what happened. Here's what we know:

May 17: The day after his 18th birthday, the shooter purchases a semi-automatic rifle at a local sporting goods store called Oasis Outback.

May 18: The gunman purchases 375 rounds of ammunition.

May 20: The gunman returns to Oasis Outback and purchases another gun.

May 24: The day of the shooting

11 a.m.: The gunman allegedly shot his 66-year-old grandmother in the head at her home. Despite her injuries, she was able to run across the street and call police, according to Steve McCraw, director of the Texas Department of Public Safety. She was rushed to a nearby hospital.

11:15 a.m.: The gunman sends a message on Facebook declaring, "I'm going to shoot an elementary school."

11:28 a.m.: McCraw says that the shooter drove about 2 miles to Robb Elementary in his grandmother's truck. When he crashed the vehicle near the school, authorities say, he exited the vehicle through a passenger side. He was carrying a rifle and a backpack full of ammunition.

According to a Thursday press conference with Texas Department of Public Safety regional director Victor Escalon, the gunman shot at two bystanders at a nearby funeral home, but they ran away uninjured.

11:32 a.m.: By all accounts, the gunman entered the school on the west side of the campus at this time, scales a fence and enters the parking lot. Shots were fired.

During Thursday's press conference, Escalon claimed that police did not engage with the shooter at this time, and he was just shooting "at the school." Previous press briefings and witness statements had claimed that police had a firefight with the gunman.

11:40 a.m.: The gunman enters west side of the school through a door that may or may not have been unlocked, Texas Department of Public Safety regional director Victor Escalon said in Thursday's press conference.

The shooter heads down a series of short hallways before entering a classroom that is connected to other classrooms. He finds one full of students. Inside, he opens fire, killing 21 people.

11:43 a.m.: Robb Elementary School posts on Facebook that the campus is on lockdown "due to gunshots in the area." The Uvalde Police Department shares similar information on Facebook.

11:44 a.m.: Local police and the district police officer enter the school and hear gunfire. Escalon says in a press conference that they initially didn't enter the classroom due to the gunfire. They fell back and called in backup, while others evacuated students and teachers.

In a conflicting account, Steven McCraw, director Texas Department of Public Safety, said that several law enforcement agencies — including Uvalde police — converged on the classroom, broke open the door, and exchanged fire with the shooter to "keep him pinned down at that location."

12:17 p.m.: The Uvalde Consolidated Independent School District announces the shooting on social media: "There is an active shooter at Robb Elementary. Law enforcement is on site," they write on Facebook. "Your cooperation is needed at this time by not visiting the campus. As soon as more information is gathered it will be shared. The rest of the district is under a Secure Status."

12:23 p.m.: Robb Elementary School parents are told by automated phone messages to pick up their children at Sgt. Willie de Leon Civic Center, about 1.3 miles away from the school. Students and teachers are transported via school bus.

Approximately 12:45 p.m.: Border patrol and tactical teams arrive at the scene. They enter the school and kill the shooter, who has been in the building for approximately an hour.

1:06 p.m.: The Uvalde Police Department initially reports the suspect is "in police custody," before amending the statement to reflect that he had been killed by officers.

The school district in Uvalde has opened an official account with First State Bank of Uvalde to support Robb Elementary families affected by the tragedy. People can send checks through the mail (payable to the "Robb School Memorial Fund") or donate money through Zelle to robbschoolmemorialfund@gmail.com. People can also donate by calling 830-356-2273.