MANKATO, Minn. — Training camp is officially underway as the Vikings held their first practice Monday afternoon on the campus of Minnesota State University, Mankato.

Only rookies, select veterans and a handful of quarterbacks have reported to camp thus far, meaning the Vikings have 40 total players in attendance. Veterans will report Wednesday.

Minnesota practiced in helmets and shorts Monday for almost two hours.

The Vikings will hold a walk-through session Tuesday morning from 10:30-11:30 a.m. before practicing from 2:45-5 p.m.

Here are three observations from Monday's session:

1. Roberson, Sinkfield see plenty of action

The pair of cornerbacks who received the most work Monday weren't even at the position as many as two years ago.

But Tre Roberson and Terrell Sinkfield have each improved over time, as evident by their performances Monday.

Roberson had the lone interception Monday as he hung with wide receiver Isaac Fruechte before hauling in a deep pass. Roberson played quarterback at Indiana and Illinois State before joining the Vikings last offseason and switching to cornerback.

Sinkfield, meanwhile, made the switch to cornerback this offseason after spending the 2016 training camp with the Vikings as a wide receiver.

The former Northern Iowa standout also had strong plays Monday, including tight coverage on wide receiver RJ Shelton that led to an incomplete pass.

Horace Richardson, who played in the secondary at Southern Methodist University, is the third cornerback currently at camp.

2. Isidora shines in camp debut

The Vikings have just offensive linemen in camp at this point, but rookie right guard Danny Isidora stood out Monday.

Isidora, a fifth-round pick out of the University of the Miami, drew praise during a run play as Vikings offensive line coach Tony Sparano encouraged the lineman to drive his feet and pushing the pile.

The 23-year-old then showed his athleticism by moving out in space to throw a block on a screen pass to rookie running back Dalvin Cook.

3. Koehn impresses with lengthy kicks

Marshall Koehn can't continue the kicking battle with Kai Forbath until the latter reports to camp later this week.

But Koehn can still make an impression on the coaching staff with a solid performance, which he put in Monday.

Koehn attempted seven kicks ranging from 33 to 50 yards and made six kicks, only missing from 39 yards. Koehn hit his final four kicks after his miss, including three from 45 yards or longer.